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)

1 files changed, 1 insertions, 1 deletions

diff --git a/sql/hostname.h b/sql/hostname.h
index 81a1d0de88d..d6137b7c260 100644
--- a/sql/hostname.h
+++ b/sql/hostname.h
@@ -168,7 +168,7 @@ extern ulong host_cache_size;
 #define RC_BLOCKED_HOST 1
 int ip_to_hostname(struct sockaddr_storage *ip_storage,
                    const char *ip_string,
-                   char **hostname, uint *connect_errors);
+                   const char **hostname, uint *connect_errors);
 
 void inc_host_errors(const char *ip_string, Host_errors *errors);
 void reset_host_connect_errors(const char *ip_string);