PS-9328: Fix for failures to shutdown server after execution of threadpool_debug test.

Problem:
========
MTR was unable to properly shutdown Percona Server after execution of
threadpool_debug test and had to resort to killing it with SIGKILL.

Analysis:
=========
threadpool_debug test simulates OOM situation when creating new THD objects
while processing new connections using threadpool plugin. This is done
by injecting and error in Thread_pool_connection_handler::add_connection()
method. The problem occurred because we failed to correctly decrement
connections count before returning from this method in case of error, like
it is done in add_connection() methods for other connection handlers.
As result during server shutdown we tried to wait indefinitely until
total number of connections will become 0, which never happened since
the connections which we failed to create due to simulated OOM were
counted as existing (even though they didn't really).

Note that the above means that failure to properly shutdown probably could
have been observed on user systems which use threadpool plugin in cases when
establishment of new connections failed due to OOM.
OTOH Percona Server/MySQL is likely to hit more serious problems in case of
OOM anyway.

Solution:
=========
Thread_pool_connection_handler::add_connection() now decrements connection
count in case of its failure as implementations of add_connection() for other
connection handlers do.

Fix is applied to both 8.0 and 8.4 as both branches are affected by this issue.

Author: dlenev

sql/threadpool_unix.cc

@@ -1197,6 +1197,7 @@ bool Thread_pool_connection_handler::add_connection(
 
   if (unlikely(!thd)) {
     channel_info->send_error_and_close_channel(ER_OUT_OF_RESOURCES, 0, false);
+    Connection_handler_manager::dec_connection_count();
     DBUG_RETURN(true);
   }
 
@@ -1206,6 +1207,7 @@ bool Thread_pool_connection_handler::add_connection(
     thd->get_protocol_classic()->end_net();
     delete thd;
     channel_info->send_error_and_close_channel(ER_OUT_OF_RESOURCES, 0, false);
+    Connection_handler_manager::dec_connection_count();
     DBUG_RETURN(true);
   }