Connection pool too slow

When running the benchmark suite using a connection pool (instead of establishing a new connection) it turns out that the connection time is significantly higher (depending on the number of connections in the pool) than performing a single connect.

This is caused by an unnecessary health check (sending MYSQL_PING) when iterating over the connections in the pool.

Instead iterating over the connections, we should keep a LRU list with unused connections, and an additional list for connection in use. When getting a connection from unused connections list, status should be checked via MYSQL_PING.