feat: add BlockingConnectionPool

[sileht]

Description

Add support for BlockingConnectionPool.

[TheKevJames]

Hi @sileht -- just wanted to let you know that my org has recently forked aredis as it appears to have become unmaintained and we've merged in this pull request (and all other open PRs against the aredis repo); in case you find it useful, the fork is here and version 2.0.0 is on PyPI with your changes.

I tweaked this change a bit by making get_connection() an awaitable in all cases, since we were planning on a hard fork and a major (breaking) version bump anyway, but otherwise left things pretty much as is since it worked perfectly for all the test cases I could think to throw at it.