fix libevent keepalive leak memery 

hhvm keepalive leak memery,we in the http.c  file of 2319 lines to join code:
 req->userdone = 1 ;

we have patched this libevent.
I have tested libevent patch,test result:
(libevent folder is patched old libevent-1.4.14.fb-changes.diff folder)
[huzg@salve3 libevent.20130829_origin]$ diff configure.in ../libevent/configure.in
[huzg@salve3 libevent.20130829_origin]$ diff event.c ../libevent/event.c
[huzg@salve3 libevent.20130829_origin]$ diff evhttp.h ../libevent/evhttp.h
[huzg@salve3 libevent.20130829_origin]$ diff http-internal.h ../libevent/http-internal.h
[huzg@salve3 libevent.20130829_origin]$ diff http.c ../libevent/http.c
2320d2319
<                       req->userdone = 1;

We only changed the http.c ,please you test this patch,thank.