Adds the end timestamp to Stat#to_s output

Also adds the request size if to_s is called before the stat is stopped
to aid in long-running request debugging.
Set the request size in the server when decoding the request from the buffer

Previously this was handled only in the socket and em server/workers,
but not in ZMQ. I pulled the code out of the implementing servers and
handle it in the base server class.
Ensure the response and request sizes are populated from the common client connector at the correct time
Use double instead of mock (deprecated in latest version of rspec)
Merge pull request #115 from localshred/bj/add_rpc_stat_timestamp

Adds ending timestamp to Stat#to_s output