Improve SPDY+HTTP integration.

Writing tests shook out a few bugs:
 - Pooling wasn't working well. We were trying to connect
   after already having been connected.
 - We weren't writing response bodies to the cache.
 - We weren't capturing the request time for the cache.
 - MockSpdyServer wasn't trimming headers.

New files in this change aren't new, they're just inner classes promoted
to top-level classes for better sharing.