Implement Fast CGI support

Split the monolithic functions preparing special variables such
as $_REQUEST or $_SERVER into multiple functions to enable feeding of
data from multiple sources. In case of FastCGI protocol, the values are
sent by the web server.

Import parts of proxygen and thrift.

Added #ifdef's to allow compilation with an unpatched version
of libevent library.

Reviewed By: @ptarjan

Differential Revision: D988737
Differential Revision: D1008756
Differential Revision: D1021808
Differential Revision: D1035231