caching: Support for proxy_cache.

This change adds support for proxy_cache integration with pagespeed with the following changes:
a) Handling cache-hit-header to bypass content handling by pagespeed.
b) pagespeed_test_conf.template with the proxy_cache configuration blocks.
c) System tests for checking hit and miss headers and purge requests.
d) Checking for proxy_cache_purge module during start-up for tests.
caching: Support for proxy_cache.

Updated to use NgxListIterator for list iteration.