Aioredis-cluster

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* add `ClusterState.slots_assigned`
* require reload cluster state for some cases with `UncoveredSlotError`

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* add `execute_timeout` for `Manager`
* improve cluster state reload logging
* reduce number of addresses to fetch cluster state
* acquire dedicate connection from pool to fetch cluster state
* extend `ClusterState` by new attributes: `state`, `state_from`, `current_epoch`

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* fix keys extraction for `XREAD` and `XREADGROUP` commands

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* make public `Address`, `ClusterNode` and `ClusterState` structs. Available by import `from aioredis_cluster import`
* `Cluster` provides some new helpful methods:
* `get_master_node_by_keys(*keys)` - return master `ClusterNode` which contains keys `keys`
* `create_pool_by_addr(addr, **kwargs)` - create connection pool by `addr` and return pool wrapped by `commands_factory` from `Cluster` constructor. By default is `aioredis_cluster.RedisCluster` instance.
* `get_cluster_state()` - return `ClusterState` instance with recent known cluster state received from Redis cluster
* `extract_keys(command_sequence)` - returns keys of command sequence
* drop `pytest-aiohttp` plugin for tests
* add `pytest-asyncio` dependency for tests
* switch `asynctest` -> `mock` library for aio tests
* drop `attrs` dependency. For Python 3.6 you need install `dataclasses`
* fix extract keys for `BLPOP`/`BRPOP` commands
* add support keys extraction for `ZUNION`, `ZINTER`, `ZDIFF`, `ZUNIONSTORE`, `ZINTERSTORE`, `ZDIFFSTORE` commands
* acquire dedicate connection from pool for potential blocking commands like `BLPOP`, `BRPOP`, `BRPOPLPUSH`, `BLMOVE`, `BLMPOP`, `BZPOPMIN`, `BZPOPMAX`, `XREAD`, `XREADGROUP`

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* README update

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* speedup crc16. Use implementation from python stdlib