Resolve "Indexer hangs intermittently"

Closes #145 (closed)

• Use connection pool for better concurrent thread access.

  Using https://github.com/brettwooldridge/HikariCP , which by default verifies connections before returning.

• Use try-with-resources where possible, prepare statements per-thread.

• Make mysql connection pool size 2 by default for cleanup threads to use a separate connection.

• Do not use transaction (autocommit false/true) for pure selects