Remove Stale connections that have not been used for idletimeout.

[aankur]

#962 Add ability to remove connections from the pool which have been stale for idletimeout period in milliseconds

[4ver]

Might be a good idea to change the following shift to pop to prevent the pool from cycling through the connections keeping em all alive.

https://github.com/mysqljs/mysql/blob/master/lib/Pool.js#L37

[adriancooney]

Any more movement on this? Our instances are hogging connections for too long and it'd be great to be able to close out those idle connections.

[akleiber]

duplicate of #2218