http: Merge branch 'http-max_connections-pr-592'

http: Merge branch 'http-max_connections-pr-592'

@jcoffland:

  "When the max connection limit is enabled and the limit is reached, the
   server will respond immediately with 503 Service Unavailable. This can
   be used to prevent servers from running out of file descriptors. This is
   better than request limiting because clients may make more than one
   request over a single connection. Blocking a request does not
   necessarily close the connection and free up a socket."

* http-max_connections-pr-592:
  test: cover evhttp max connections
  Added evhttp max simultaneous connection limiting