docs: updated protocol.md missed in #158
nsqd: allow configurable max heartbeat interval
go-nsq: RDY state updates:

* bootstrap RDY count to 1
* use feature negotiation to get server max RDY count
* never send RDY count > server max RDY count
Merge pull request #175 from mreiferson/config_rdy_175

nsqd: allow configurable max RDY count