Add connection-specific rate-limiting function.

- added evhtp_connection_set_ratelimit() which is just a wrapper around
  libevent's rate-limiting api.
Fix double free for evthr at shutdown

If you use threads, than you get this segfault.

There was two calls to evthr_free() on shutdown:
- _evthr_loop() (droped in this patch)
- evthr_pool_free()
Release 1.2.6, see ChangeLog for details

This is primarily a hotfix release. A few commits have been cherry-picked from
develop and do not include some of the resume fixes as they have not been tested
thoroughly as of yet.