double-forward-declaration: remove a duplicate forward declaration
Merge pull request #423 from pagespeed/oschaaf-double-forward-declaration

double-forward-declaration: remove a duplicate forward declaration
fetch-rate-limiting: add rate limiting for background fetches

Limits the number of simultaneous background fetches performed.
Requires statistics to be turned on.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/426
request: allow alternate request methods when rewriting html
Merge pull request #427 from pagespeed/oschaaf-rate-limiting

fetch-rate-limiting: add rate limiting for background fetches
Merge pull request #428 from pagespeed/jefftk-head-get

request: allow alternate request methods when rewriting html
merge: rate controlled fetcher