HHVM changes

We use libevent for our webserver when serving facebook.com. Currently we are on libevent1.4.14b [1] but I'm trying to see if we can use libevent2. In doing so, I ported our patch to 2.0.21. Would you like these changes upstream?

I wasn't the original author of this patch, but as far as I understand, it:

1) Exposes the connection count
2) Adds a connection limit
3) Handles malformed HTTP requests
4) Adds WebDAV verb support and exposes the passed verb
5) Adds an asyncronous output API
6) Allows the backlog to be configured

On top of this, we also use an SSL patch which I haven't yet gotten ported to libevent2 (or figured out if libevent already supports everything we need).

[1] https://github.com/facebook/hhvm/wiki/Building-and-installing-HHVM-on-Ubuntu-13.10 for example