PREBUILT->TABLE->N_MYSQL_HANDLES_OPENED == 1

ANALYSIS:
=========

Adding unique index to a InnoDB table which is locked as
mutliple instances may trigger an InnoDB assert.

When we add a primary key or an unique index, we need to
drop the original table and rebuild all indexes. InnoDB
expects that only the instance of the table that is being
rebuilt, is open during the process. In the current
scenario we have opened multiple instances of the table.
This triggers an assert during table rebuild.
'Locked_tables_list' encapsulates a list of all
instances of tables locked by LOCK TABLES statement.

FIX:
===
We are now temporarily closing all the instances of the
table except the one which is being altered and later
reopen them via Locked_tables_list::reopen_tables().