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_base.cc

@@ -1233,7 +1233,8 @@ bool close_temporary_tables(THD *thd)
       */
       for (;
            table && is_user_table(table) &&
-             tmpkeyval(thd, table) == thd->variables.pseudo_thread_id &&
+             (ulong) tmpkeyval(thd, table) ==
+             (ulong) thd->variables.pseudo_thread_id &&
              table->s->db.length == db.length() &&
              memcmp(table->s->db.str, db.ptr(), db.length()) == 0;
            table= next)