Generate RDB with Functions only via redis-cli --functions-rdb (#9968)

This is needed in order to ease the deployment of functions for ephemeral cases, where user
needs to spin up a server with functions pre-loaded.

#### Details:

* Added `--functions-rdb` option to _redis-cli_.
* Functions only rdb via `REPLCONF rdb-filter-only functions`. This is a placeholder for a space
  separated inclusion filter for the RDB. In the future can be `REPLCONF rdb-filter-only
  "functions db:3 key-patten:user*"` and a complementing `rdb-filter-exclude` `REPLCONF`
  can also be added.
* Handle "slave requirements" specification to RDB saving code so we can use the same RDB
  when different slaves express the same requirements (like functions-only) and not share the
  RDB when their requirements differ. This is currently just a flags `int`, but can be extended to
  a more complex structure with various filter fields.
* make sure to support filters only in diskless replication mode (not to override the persistence file),
  we do that by forcing diskless (even if disabled by config)

other changes:
* some refactoring in rdb.c (extract portion of a big function to a sub-function)
* rdb_key_save_delay used in AOFRW too
* sendChildInfo takes the number of updated keys (incremental, rather than absolute)

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

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