MDEV-6150 Speed up connection speed by moving creation of THD to new thread

Creating a CONNECT object on client connect and pass this to the working thread which creates the THD.
Split LOCK_thread_count to different mutexes
Added LOCK_thread_start to syncronize threads
Moved most usage of LOCK_thread_count to dedicated functions
Use next_thread_id() instead of thread_id++

Other things:
- Thread id now starts from 1 instead of 2
- Added cast for thread_id as thread id is now of type my_thread_id
- Made THD->host const (To ensure it's not changed)
- Removed some DBUG_PRINT() about entering/exiting mutex as these was already logged by mutex code
- Fixed that aborted_connects and connection_errors_internal are counted in all cases
- Don't take locks for current_linfo when we set it (not needed as it was 0 before)

sql/sql_parse.cc

@@ -844,12 +844,13 @@ end:
   delete thd;
 
 #ifndef EMBEDDED_LIBRARY
-  thread_safe_decrement32(&thread_count);
+  DBUG_ASSERT(thread_count == 1);
   in_bootstrap= FALSE;
-
-  mysql_mutex_lock(&LOCK_thread_count);
-  mysql_cond_broadcast(&COND_thread_count);
-  mysql_mutex_unlock(&LOCK_thread_count);
+  /*
+    dec_thread_count will signal bootstrap() function that we have ended as
+    thread_count will become 0.
+  */
+  dec_thread_count();
   my_thread_end();
   pthread_exit(0);
 #endif
@@ -954,7 +955,6 @@ bool do_command(THD *thd)
   if(!thd->skip_wait_timeout)
     my_net_set_read_timeout(net, thd->variables.net_wait_timeout);
 
-
   /*
     XXX: this code is here only to clear possible errors of init_connect. 
     Consider moving to init_connect() instead.
@@ -7279,7 +7279,6 @@ void mysql_parse(THD *thd, char *rawbuf, uint length,
             and Query_log_event::print() would give ';;' output).
             This also helps display only the current query in SHOW
             PROCESSLIST.
-            Note that we don't need LOCK_thread_count to modify query_length.
           */
           if (found_semicolon && (ulong) (found_semicolon - thd->query()))
             thd->set_query_inner(thd->query(),