The length of the carbon-relay's per-destination queue is not visible.

This patch adds
carbon.relays.<hostname>.destinations.<dest-host>.relayMaxQueueLength,
a counter that records the maximum queue length for that destination
during the reporting period.
Remove an un-needed attempt to send a instrumentation stat.
Fix relayMaxQueueLength instrumentation to be in factory, not protocol
Merge pull request #138 from bitprophet/better-relay-queue-size-0.9.x

Relay queue size metrics