[YugabyteDB] Improve locking mechanism during migrations

[ashetkar]

This plugin replaced the use if PG Advisory locks during migrations with a mechanism using SELECT ... FOR UPDATE. This PR further improves this locking mechanism as explained below.

• Ensure only the owner of the lock unlocks it
  • There was a possibility that another thread might set the locked column to false in the YB_FLYWAY_LOCK_TABLE table even if it didn't set it to true earlier.
  • To avoid this, changed column locked of type boolean to lock_id of type bigint and checking its value while resetting it.
• Ensure the lock is not held indefinitely
  • Added a timestamp column in the lock table
  • If this (current_time - timestamp) is more than a specified ttl value (default 5min and configurable via a system property), the lock is reset.
• Tested the changes with existing tests in flyway-tests

[ashetkar]

cc @Barry-RG Please take look. Thanks