Implements connect as a FSM and fixes cancellation

[anarthal]

Implements redis_stream::async_connect as a FSM
Adds per-operation cancellation handling code
Adds tests

[anarthal]

This is an early prototype yet. I've started implementing #319 and it was getting too big and messy around redis_stream::async_connect. This implements the required code to handle per-operation cancellation correctly in async_connect, while covering this code with tests.

[anarthal]

This should fix the last race conditions on cancellation and add some coverage.

[mzimbres]

Hi Ruben, thanks. This is something I forgot to ask you before: can you please rebase on top of develop before opening a PR and merging? This results in simpler history and it is easier for me to review the PR locally. I think it will be too complex for this PR so we can do this for the next PRs.

[anarthal]

I think this one is actually good - it's just ultra segmented in terms of commits, but they're all part of the PR. I always do squash merging when merging, so these won't appear in history - just a big commit replacing them.

I did something wrong in the cancel_after PR which I think I've solved now.

[anarthal]

(looks like I commited once per test and wrote about 20 tests, so... :))