[DO NOT MERGE] Improve the lock management

[ashetkar]

• Ensure only the owner unlocks the lock
  • There is 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 boolean to lock_id 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 60000ms and configurable via a system property), lock is reset.
• Tested the changes with existing tests in flyway-tests
• Once reviewed, this PR will be raised against the upstream master.

[kneeraj]

Did an offline review with Amogh:
Gave the following suggestions which are now taken care of. So approving.

Suggestions given:

1. There was a blind drop of lock table in the initialisation code which would have failed an ongoing migration. A concurrency issue.
2. Advised to use random long for id in order to make the clash of id used for uniqueness negligible. It was int earlier.
3. Increase the max consideration time of a migration life from 1 min to 5 min. In fact have advised to test with large migrations in a real distributed setup as migrations are real ddl executions and a big one can take a lot of time.
4. Put some code comments around the logic.

[ashetkar]

The PR is raised with the upstream repo flyway#76