cleanup: string sys_var types

Merge sys_var_charptr with sys_var_charptr_base, as well as merge
Sys_var_session_lexstring into Sys_var_lexstring. Also refactored
update methods of sys_var_charptr accordingly.

Because the class is more generic, session_update() calls
sys_var_charptr::session_update() which does not assume a buffer field
associated with THD, but instead call strdup/free, we get rid of
THD::default_master_connection_buff accordingly. This also makes THD
smaller by ~192 bytes, and there can be many thousands of concurrent
THDs.

sql/sql_class.h

@@ -3750,7 +3750,6 @@ public:
     This is used for taging error messages in the log files.
   */
   LEX_CSTRING connection_name;
-  char       default_master_connection_buff[MAX_CONNECTION_NAME+1];
   uint8      password; /* 0, 1 or 2 */
   uint8      failed_com_change_user;
   bool       slave_thread;