feature: host fallback support

[aMahanna]

Closes #179

A (working) first-draft implementation for host fallback support.

Requires feedback

What's new:

• BaseConnection.process_request()
  • Given a request, process it until a valid response is provided, or the max amount of tries has been reached.
  • host_index is still being chosen using the resolver's get_host_index method.
• HostResolver.max_tries : int
  • A limit to the amount of attempts made to execute the request. Value varies based on the resolver type
• HostResolver.host_count : int
  • The number of hosts provided
• RandomHostResolver.get_host_index()
  • Reworked to keep track of previously chosen host indexes, to make sure that the next chosen host_index is not one of them.

[joowani]

Looks like a good start. I left some comments, some of which may not need to be addressed immediately if you are looking to do incremental changes.

Also, I'm not sure how much of this overlaps with active failover in ArangoDB (which python-arango does not have support for yet). I started a thread on it in the past but never got a chance to work on it. Hopefully that helps you in the future.

[joowani]

Lgtm. Thanks for your contribution!