early quorum responses

Allow the proxy to respond to many types of requests as soon as it has a
quorum.  This can help speed up responses (without changing the results),
especially when one node is acting up.

I had to fix a few unit tests that no longer match the backend http requests
made by our proxy.

Change-Id: Ieb070dc3019e217e717b96154a7a809409bf40a5
Merge "early quorum responses"