MDEV-11853: semisync thread can be killed after sync binlog but before ACK in the sync state

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>

sql/sql_class.h

@@ -4838,6 +4838,14 @@ public:
   bool is_binlog_dump_thread();
 #endif
 
+  /*
+    Indicates if this thread is suspended due to awaiting an ACK from a
+    replica. True if suspended, false otherwise.
+
+    Note that this variable is protected by Repl_semi_sync_master::LOCK_binlog
+  */
+  bool is_awaiting_semisync_ack;
+
   inline ulong wsrep_binlog_format() const
   {
     return WSREP_BINLOG_FORMAT(variables.binlog_format);