Asyncio-redis

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Fixes:
- Protocol was in a faulty state after a transaction failure.
- Pubsub did not work with BytesEncoder.

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Fixes:
- Fixed broken psubscribe.

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Fixes:
- HiRedisProtocol implementation has been improved. (Cleaner code and a little
faster.)

Backward-incompatibilities:
- Some arguments of Connection, Pool and RedisProtocol are now keyword-only.
- Made hiredis on optional dependency. (Using extra_required in the setup.)

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New features:
- Added `protocol_class` parameter for Pool and Connection
- Added experimental `HiRedisProtocol` class.

Fixes:
- Improving the performance of multibulk replies (Especially _asset, _asdict
and _aslist methods should be significantly faster.)

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Fixes:
- Circular reference between `Connection` and `Protocol` was never cleaned up
because of __del__.
- Memory leaks in unit tests were fixed.

Backward-incompatibilities:
- Connection is not closed anymore in __del__, like in version 0.12.4, but
instead the `Connection` and `Pool` classes got an explicit close method.

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New features:
- Unix socket support. (When port=3, we suppose unix socket connection.)

Fixes:
- Closing of transport in Connection.__del__ and unittests.