Reduce fd usage, support convert_meta_tags

This reduced fd usage in NgxBaseFetch, and fixes support for convert_meta_tags

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/56

Squash merge of chaizhenhua's https://github.com/pagespeed/ngx_pagespeed/pull/241

Conflicts:
	src/ngx_pagespeed.cc
Reduce fd usage & support convert_meta_tags

This reduced fd usage in NgxBaseFetch, and fixes support for convert_meta_tags

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/56

Squash merge of chaizhenhua's https://github.com/pagespeed/ngx_pagespeed/pull/241

Conflicts:
	src/ngx_pagespeed.cc
Merge commit '3927541c27c2376b543e046d9ed80bf6a162fc8c'