PROBLEM

Each memcached client connection takes a MDL lock which it releases only when the
connection terminates or it switches table. Becasue of this MDL lock FLUSH TABLE
WITH READ LOCK command hangs.

FIX

Introduced a internal variable called "release_mdl_lock" which will be monitored by
background thread and set/reset by FTWRL/unlock tables; when this variable is set
this following operations will be initiated

1) Any write operation will be forced to close and then open the table using
   handler functions.Closing the table will ensure that the mdl lock are released
   and the FTWRL can obtain the global read lock. Trying to reopen the table will
   hang.The write opeartion will succeed once unlock tables is executed.

2) Read operations are not forced to reopen the table, this is because we want read operations
   to continue during FTWRL.

3) Background thread will detect connections which are not in use (idle) and commit transactions
   assocaited with them and close all the table handles