Problem:
In debug builds, there is a chance that an out-of-bounds
read is performed when tables are locked in
LTM_PRELOCKED_UNDER_LOCK_TABLES mode. It can happen because
the debug code uses enum values as index for an array of
mode descriptions, but it only takes into consideration 3
out of 4 of the enum values.

Fix:
This patch fixes it by implementing a getter for the enum which
returns a string representation of the enum,
effectively removing the out-of-bounds read.

Moreover, it also fixes the lock mode descriptions that
would be print out in debug builds.