In-flight ZCL response handlers aren't being cleared on timeout

[elliots]

A bug in this code:
https://github.com/ninjablocks/node-zigbee/blob/master/lib/zcl/ZCLClient.js#L67-L79

Leading to this message being thrown:
https://github.com/ninjablocks/node-zigbee/blob/master/lib/zcl/ZCLClient.js#L46

@thatguydan