Use blocking connection pool

[stinovlas]

Closes #53.

I decided not to add timeout argument at the moment, because any unknown argument passed to constructor is passed to the BlockedConnectionPool (including timeout if user wishes to customize it).

BlockingConnectionPool default is 20 seconds. If you think it's too high, we can still customize that value without changing constructors themselves, like this:

if "timeout" not in connection_kwargs:
    connection_kwargs["timeout"] = 3  # or other sensible default

TODO

• [x] replace ConnectionPool with BlockingConnectionPool
• [x] add connection_kwargs (and max connections) to result backends
• [x] add tests
• [x] update documentation

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (09e835e) 96.31% compared to head (7cfb6f7) 96.31%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #54 +/- ## ======================================== Coverage 96.31% 96.31% ======================================== Files 7 7 Lines 217 217 ======================================== Hits 209 209 Misses 8 8 ```

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[s3rius]

Seems good for me.