Bug#23236404 QUERY REWRITE PLUGIN SUFFERS SCALABILITY ISSUES

Problem:
========
Audit plugins are being acquired on the first use. Releasing plugins after execution
of each query and reacquiring them again significantly affects scalability.

Fix:
====
Audit plugins are being acquired on the first use, and released when the connection ends.

Reviewed-by:
============
Georgi Kodinov <georgi.kodinov@oracle.com>
Ramil Kalimullin <ramil.kalimullin@oracle.com>

Author: maras007

mysql-test/t/version_token.test

@@ -207,6 +207,7 @@ connection con1;
 set @@version_tokens_session= "token100=def;;;      token2  =   abc    ;;;;;";
 --error ER_VTOKEN_PLUGIN_TOKEN_NOT_FOUND
 create table t1 (c1 int);
+disconnect con1;
 
 -- echo
 -- echo # Connection: default

sql/conn_handler/connection_handler_one_thread.cc

@@ -84,7 +84,6 @@ bool One_thread_connection_handler::add_connection(Channel_info* channel_info)
     delete channel_info;
     while (thd_connection_alive(thd))
     {
-      mysql_audit_release(thd);
       if (do_command(thd))
         break;
     }

sql/conn_handler/connection_handler_per_thread.cc

@@ -297,7 +297,6 @@ extern "C" void *handle_connection(void *arg)
     {
       while (thd_connection_alive(thd))
       {
-        mysql_audit_release(thd);
         if (do_command(thd))
           break;
       }

sql/sql_plugin.cc

@@ -4185,6 +4185,9 @@ bool Sql_cmd_install_plugin::execute(THD *thd)
   bool st= mysql_install_plugin(thd, &m_comment, &m_ident);
   if (!st)
     my_ok(thd);
+#ifndef EMBEDDED_LIBRARY
+  mysql_audit_release(thd);
+#endif
   return st;
 }
 
@@ -4194,5 +4197,8 @@ bool Sql_cmd_uninstall_plugin::execute(THD *thd)
   bool st= mysql_uninstall_plugin(thd, &m_comment);
   if (!st)
     my_ok(thd);
+#ifndef EMBEDDED_LIBRARY
+  mysql_audit_release(thd);
+#endif
   return st;
 }