Reuse THD for new user connections

- To ensure that mallocs are marked for the correct THD, even if it's
  allocated in another thread, I added the thread_id to the THD constructor
- Added st_my_thread_var to thr_lock_info_init() to avoid a call to my_thread_var
- Moved things from THD::THD() to THD::init()
- Moved some things to THD::cleanup()
- Added THD::free_connection() and THD::reset_for_reuse()
- Added THD to CONNECT::create_thd()
- Added THD::thread_dbug_id and st_my_thread_var->dbug_id. These are needed
  to ensure that we have a constant thread_id used for debugging with a THD,
  even if it changes thread_id (=connection_id)
- Set variables.pseudo_thread_id in constructor. Removed not needed sets.

sql/sql_profile.cc

@@ -328,14 +328,28 @@ PROFILING::PROFILING()
 }
 
 PROFILING::~PROFILING()
+{
+  restart();
+}
+
+/*
+  Restart profiling from scratch
+*/
+
+void PROFILING::restart()
 {
   while (! history.is_empty())
     delete history.pop();
 
   if (current != NULL)
     delete current;
+  /* Ensure that profiling object can be reused */
+  profile_id_counter= 1;
+  current= NULL;
+  last= NULL;
 }
 
+
 /**
   Throw away the current profile, because it's useless or unwanted
   or corrupted.
@@ -675,6 +689,6 @@ int PROFILING::fill_statistics_info(THD *thd_arg, TABLE_LIST *tables, Item *cond
 
 void PROFILING::reset()
 {
-  enabled= thd->variables.option_bits & OPTION_PROFILING;
+  enabled= (thd->variables.option_bits & OPTION_PROFILING) != 0;
 }
 #endif /* ENABLED_PROFILING */