native fetcher: Support http keep-alive

Based on @dinic his work, add keep-alive support for the native fetcher.
Adds a new option, usable at the http{} level in configuration:

pagespeed NativeFetcherMaxKeepaliveRequests 50;

The default value is 100 (aligned to nginx). Setting the value to 1 turns off
keep-alive requests altogether).

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
You had a typedef for Pool<NgxConnection> above.