Close connectons async.
Why: When db goes down, it can cause HikariCP to block on getConnection
more than the allowed connectionTimeout, depending on jdbc driver
timeout setting. In some cases, this could be a long time.
Added a test that shows this behaviour. The test will fail w/o the
changes to HikariPool.
Merge pull request #147 from mihai-chezan/dev

Close connectons async.