Oschaaf native fetcher keepalive

A few changes (098d3b8) on top of @dinic his work on keep-alive support for the native fetcher.
Most notable changes:
- Request keep-alive by adding the appropriate request header
- Fixes connections getting reused while they are servicing other requests:
- Remove connection from the pool of available connections for keepalive when applicable
- Disable keepalive in more appropriate situations
- Response parsing fixes
- Remove connections that timeout from the k.a. pool
- Add a few sanity (D)CHECKS
- Emit debug messages for traceability

Note that this change does not reformat the indenting within the net_instaweb namespace
of ngx_fetch.cc, as that would make it very hard to assess what has changes from the diff.

Note: The 'PageSpeed CSS loaded in fallback mode is always chunked' tests must be disabled
when testing (currently fails on master without this change as well).
I wish the server header was `Connection: Keep-Alive [time in ms]` like `Connection: Keep-Alive 5000`.

Without that, though, we are sometimes going to pick connections that have just closed or will close very soon.  How common this is will depend on the latency between us and the remote server.  In the most common case, though, we expect these connections to stay hot, and then it should work very well.

Maybe we just fix this if it turns out to be a problem?