block_on usage stalls my application

[RAnders00]

The fact that the Drop implementation uses a block_on executor strategy, combined with the fact that it blocks until it gets hold of a mutex, seems to be a problem. I had to fork the project and change to a tokio::spawn-based approach that would return the connection to the pool asynchronously.
My app is retrieving a connection on every web request, and it was basically never responding after 10 requests (the configured minimum pool size). I debugged it to the point to figure out it was infinitely waiting to get() a connection from the pool.
I did not dig into it further than that quick "bandage fix". Maybe this is something that needs to be addressed generally?

This is the relevant commit: https://github.com/RAnders00/bb8/commit/e456d4c8812adc445c85852b575f728981cb5e0a

[RAnders00]

Also I'm not sure how using a futures single-threaded executor combines with the standard tokio multi-threaded executor I am using. That could be a problem too.

[djc]

There's no longer any block_on in the Drop implementation. If you're still interested, please test current master and give me feedback.