Check consumers for pending messages before deletion

[hlubek]

This fixes the original issue by checking consumer info first but still has a chance of a race condition (in theory). I also added a test which can reproduce the issue with lost messages without that fix.

Having an additional loop to handle consumer deletion would need some more changes though, so I went for the easier solution for now.

Fixes #24

[minghsu0107]

@hlubek Great job! I've made some minor improvements to your code and also included configuration options for the subscriber. Take a look at the changes I've made in this branch: https://github.com/ThreeDotsLabs/watermill-redisstream/tree/pr-24

Please feel free to reference my code and integrate it with yours. We can then update the new version in this PR.

[hlubek]

@minghsu0107 Thanks! I just integrated your changes.

[minghsu0107]

LGTM