- Description:
  - Before 10.3.8 semisync was a plugin that is built into the server with
    MDEV-13073,starting with commit cbc71485e24c31fc822277625512e55c2a8b650b.
    There are still some usage of `rpl_semi_sync_master` in mtr.
Note:
  - To recognize the replica in the `dump_thread`, replica is creating
    local variable `rpl_semi_sync_slave` (the keyword of plugin) in
    function `request_transmit`, that is catched by primary in
    `is_semi_sync_slave()`. This is the user variable and as such not
    related to the obsolete plugin.

 - Found in `sys_vars.all_vars` and `rpl_semi_sync_wait_point` tests,
   usage of plugins `rpl_semi_sync_master`, `rpl_semi_sync_slave`.
   The former test is disabled by default (`sys_vars/disabled.def`)
   and marked as `obsolete`, however this patch will remove the queries.

- Add cosmetic fixes to semisync codebase

Reviewer: <brandon.nesterenko@mariadb.com>
Closes PR #2528, PR #2380

Problem:
========
If a primary is shutdown during an active semi-sync connection
during the period when the primary is awaiting an ACK, the primary
hard kills the active communication thread and does not ensure the
transaction was received by a replica. This can lead to an
inconsistent replication state.

Solution:
========
During shutdown, the primary should wait for an ACK or timeout
before hard killing a thread which is awaiting a communication. We
extend the `SHUTDOWN WAIT FOR SLAVES` logic to identify and ignore
any threads waiting for a semi-sync ACK in phase 1. Then, before
stopping the ack receiver thread, the shutdown is delayed until all
waiting semi-sync connections receive an ACK or time out. The
connections are then killed in phase 2.

Notes:
 1) There remains an unresolved corner case that affects this
patch. MDEV-28141: Slave crashes with Packets out of order when
connecting to a shutting down master. Specifically, If a slave is
connecting to a master which is actively shutting down, the slave
can crash with a "Packets out of order" assertion error. To get
around this issue in the MTR tests, the primary will wait a small
amount of time before phase 1 killing threads to let the replicas
safely stop (if applicable).
 2) This patch also fixes MDEV-28114: Semi-sync Master ACK Receiver
Thread Can Error on COM_QUIT

Reviewed By
============
Andrei Elkin <andrei.elkin@mariadb.com>

Problem:
========
When both semi-sync and slave compression are enabled, the numbering
on packet headers can become out of sync between the primary and
replica servers. More specifically, after the master flushes its
write, it should increment the counters that track packets. The
bug is such that the master only updates the normal packet counter
and leaves the compressed packet counter alone.

Solution:
========
After the master flushes, additionally increment the compressed
packet counter.

Reviewed By:
============
Andrei Elkin: <andrei.elkin@mariadb.com>

All changes (except one) is of type
thd->transaction.  -> thd->transaction->

thd->transaction points by default to 'thd->default_transaction'
This allows us to 'easily' have multiple active transactions for a
THD object, like when reading data from the mysql.proc table