ACCESS::CREATE_THD()/DROP_THD FUNCTIONS

Problem: Current_thd is costly operation and it is getting
called in Rpl_info_table_access:create_thd/drop_thd functions
many times and it can be avoided.

Analysis: 
In case of RPL_INFO_REPOSITORY=TABLE, Server try to open and
close repository table. In the process of opening and closing
the table, there are few calls to costly macro current_thd
which can be avoided by changing the logic little.

Fix: Replacing save_current_thd logic with 'thd_created'
(bool) variable.