Client eviction (#8687)

### Description
A mechanism for disconnecting clients when the sum of all connected clients is above a
configured limit. This prevents eviction or OOM caused by accumulated used memory
between all clients. It's a complimentary mechanism to the `client-output-buffer-limit`
mechanism which takes into account not only a single client and not only output buffers
but rather all memory used by all clients.

#### Design
The general design is as following:
* We track memory usage of each client, taking into account all memory used by the
  client (query buffer, output buffer, parsed arguments, etc...). This is kept up to date
  after reading from the socket, after processing commands and after writing to the socket.
* Based on the used memory we sort all clients into buckets. Each bucket contains all
  clients using up up to x2 memory of the clients in the bucket below it. For example up
  to 1m clients, up to 2m clients, up to 4m clients, ...
* Before processing a command and before sleep we check if we're over the configured
  limit. If we are we start disconnecting clients from larger buckets downwards until we're
  under the limit.

#### Config
`maxmemory-clients` max memory all clients are allowed to consume, above this threshold
we disconnect clients.
This config can either be set to 0 (meaning no limit), a size in bytes (possibly with MB/GB
suffix), or as a percentage of `maxmemory` by using the `%` suffix (e.g. setting it to `10%`
would mean 10% of `maxmemory`).

#### Important code changes
* During the development I encountered yet more situations where our io-threads access
  global vars. And needed to fix them. I also had to handle keeps the clients sorted into the
  memory buckets (which are global) while their memory usage changes in the io-thread.
  To achieve this I decided to simplify how we check if we're in an io-thread and make it
  much more explicit. I removed the `CLIENT_PENDING_READ` flag used for checking
  if the client is in an io-thread (it wasn't used for anything else) and just used the global
  `io_threads_op` variable the same way to check during writes.
* I optimized the cleanup of the client from the `clients_pending_read` list on client freeing.
  We now store a pointer in the `client` struct to this list so we don't need to search in it
  (`pending_read_list_node`).
* Added `evicted_clients` stat to `INFO` command.
* Added `CLIENT NO-EVICT ON|OFF` sub command to exclude a specific client from the
  client eviction mechanism. Added corrosponding 'e' flag in the client info string.
* Added `multi-mem` field in the client info string to show how much memory is used up
  by buffered multi commands.
* Client `tot-mem` now accounts for buffered multi-commands, pubsub patterns and
  channels (partially), tracking prefixes (partially).
* CLIENT_CLOSE_ASAP flag is now handled in a new `beforeNextClient()` function so
  clients will be disconnected between processing different clients and not only before sleep.
  This new function can be used in the future for work we want to do outside the command
  processing loop but don't want to wait for all clients to be processed before we get to it.
  Specifically I wanted to handle output-buffer-limit related closing before we process client
  eviction in case the two race with each other.
* Added a `DEBUG CLIENT-EVICTION` command to print out info about the client eviction
  buckets.
* Each client now holds a pointer to the client eviction memory usage bucket it belongs to
  and listNode to itself in that bucket for quick removal.
* Global `io_threads_op` variable now can contain a `IO_THREADS_OP_IDLE` value
  indicating no io-threading is currently being executed.
* In order to track memory used by each clients in real-time we can't rely on updating
  these stats in `clientsCron()` alone anymore. So now I call `updateClientMemUsage()`
  (used to be `clientsCronTrackClientsMemUsage()`) after command processing, after
  writing data to pubsub clients, after writing the output buffer and after reading from the
  socket (and maybe other places too). The function is written to be fast.
* Clients are evicted if needed (with appropriate log line) in `beforeSleep()` and before
  processing a command (before performing oom-checks and key-eviction).
* All clients memory usage buckets are grouped as follows:
  * All clients using less than 64k.
  * 64K..128K
  * 128K..256K
  * ...
  * 2G..4G
  * All clients using 4g and up.
* Added client-eviction.tcl with a bunch of tests for the new mechanism.
* Extended maxmemory.tcl to test the interaction between maxmemory and
  maxmemory-clients settings.
* Added an option to flag a numeric configuration variable as a "percent", this means that
  if we encounter a '%' after the number in the config file (or config set command) we
  consider it as valid. Such a number is store internally as a negative value. This way an
  integer value can be interpreted as either a percent (negative) or absolute value (positive).
  This is useful for example if some numeric configuration can optionally be set to a percentage
  of something else.

Co-authored-by: Oran Agra <oran@redislabs.com>

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