Implement a pool leak detection

There is different issues with pool leak. (see from https://github.com/mariadb-corporation/mariadb-connector-nodejs/issues/93)
Implementation must have a new option that when enable will permit to detect (log to console) possible connection leak.

When a connection is borrowed from pool and this timeout is reached, a message must be logged to console indicating a possible connection leak. Another message must tell if the possible logged leak has been released.