Add transaction support for redis distributed

[EiNSTeiN-]

I am trying to use watch and multi with Redis::Distributed. My code looks like this:

r.watch("{uuid}:data", "{uuid}:version") do
  current = r.get("{uuid}:version")
  r.multi do
    r.set("{uuid}:data", new_data)
    r.set("{uuid}:version", current + 1)
  end
end

Given all keys can be tagged, they will all end up on the same node, so this should work with Redis::Distributed without issue. An additional assumption is that the caller will use the block syntax for watch which is thread safe, or will handle thread safety in another way outside of Redis::Distributed.

I implemented that by keeping a copy of the key tag or the key if the key itself isn't tagged, and ensure that all commands are issued with the same key or key tag until one of these things happen:

1. exec, unwatch or discard is explicitly called
2. exec is implicitly called at the end of the multi block
3. unwatch is implicitly called when a StandardError is raised within a watch block

[supercaracal]

CI tests of the repository tend to be flaky now. These irrelevant failures are ignorable. They will be resolved by #930.

[byroot]

That's not good enough, you want to reset @watch_key from an ensure block. Here you only reset it on error.

[byroot]

Hum, actually scratch that, it seems watch do is not expected to unwatch... that's a very weird behavior :/

nevermind

[byroot]

Ok, so ignore my previous comment. I wasn't so familiar with the watch interface. Turns out the unwatch has to be manual, sounds like a terrible API to me but hey...