Add support for shard latency checking (#155)
Add support for shard latency checking (#155)

* Add support for shard latency checking

- Each shard now stores the last time it sent a heartbeat, and when it was last acknowledged
- Shard getter returns latency if both values exist

* Add documentation
- Oops, forgot

* Requested changes
- Return Infinity if latency can't currently be calculated
- Clarify docs a bit

abalabahaha/eris,Flyy-y/eris,briantanner/eris,Datamats/eris