h2load: Override user-agent with -H option
h2load: Override user-agent with -H option

This commit allows user to override user-agent with -H option.  We
also enabled custom header support for http/1.1 requests.  We also did
minor optimizations (std::vector::reserve).

icing/nghttp2,icing/nghttp2,icing/nghttp2,icing/nghttp2,icing/nghttp2,icing/nghttp2,icing/nghttp2