introduce heartbeat check to fallback host support

[aMahanna]

We are currently processing the client request to a given host, without knowing whether it is alive or not.

• If the host happens to be down, this can lead to a long wait before the ConnectionAbortedError is raised
  (see current implementation of process_request)

We should instead heartbeat the host first, and if it is alive, process the request.

• This can be simply achieved by running a requests.get() on the host selected by our HostResolver with a set timeout parameter.

[aMahanna]

No longer applicable, not a valid solution to the the issue at hand. Will come back & search for an alternative..