Add a debug-only field MDL_context::lock_warrant. This field can be set
to the MDL context different from the one the current execution is done in.

The lock warrantor has to hold an MDL for at least a duration of a table
lifetime.

This is needed in the subsequent commit so that the shared MDL acquired by
the InnoDB purge_coordinator_task can be shared by purge_worker_task
that access index records that include virtual columns.

Reviewed by: Vladislav Vaintroub