Update system test to be compatible with a helper-script update that makes start_test clear WGET_ARGS, making test stanzas more independent
Merge pull request #782 from pagespeed/jmarantz-start-test-resets-wget-args

Assume WGET_ARGS is cleared on every start_test
use fgrep for counting ".pagespeed.", rather than grep with backslashed dot, which matches too much
Merge pull request #784 from pagespeed/jmarantz-fgrep-for-counts

use fgrep for counting ".pagespeed."
Override the method used to specify the format for conf files, so we can put an accurate error message in the admin GUI, and test it
Merge pull request #785 from pagespeed/jmarantz-error-option-format

Implement NgxServerContext::FormatOption so we can indicate nginx formatting in error messages
native-fetcher: support keep-alive requests

Based on @dinic his work, add keep-alive support for the native fetcher.
Adds one new option, only usable at the http{} level in configuration:

pagespeed NativeFetcherMaxKeepaliveRequests 50;

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