Only export public symbols.
Export evhtp_connection_set_ratelimit
Uncomplexify evthr - huge performance boost.
Do backlogs matter for evthr? I am thinking not.
Remove all the stupid backlog stuff.
Proposed changes for request pause/resume (pipelined)

- When a request is paused, both EV_READ AND EV_WRITE are disabled.

- When a user wants to resume the request, they must call request_resume AFTER
  send_reply (to make sure there is data in the bufferevent's output)

- The resume function will then check to see if there is data waiting to be
  sent. If true, ONLY the EV_WRITE flag is set on the bufferevent

- Once all data has been written, the EV_READ flag is re-enabled, and everything
  should work as usual.

REMEMBER: resume needs to be called AFTER evhtp_send_reply()!
Remove dead code from evthr
Remove dead declarations in evthr.h