Close db connection whenever failed to retrieve lock to prevent connection leak

[jiahui-grabtaxi]

We used this library in one of our application to prevent race condition when two cron jobs are running at the same time.
Our cron job runs every hour, the one that failed to retrieve the lock will always have its connection opened.
The connection count increases until the connection pool exhausted and our application crashed.

Please consider merging this into the main branch 😄

[sanketplus]

thank you for the fix :)