Aioredis-cluster

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* remove `state_reload_frequency` from `ClusterManager`. `state_reload_interval` now is one relevant option for state auto reload
* default `state_reload_interval` increased and now is 300 seconds (5 minutes)
* commands registry loads only once, on cluster state initialize
* improve failover. First connection problem cause retry to random slot replica
* improve python3.9 support
* default `idle_connection_timeout` now is 10 minutes

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* fix `aioredis.locks.Lock` issue (https://github.com/aio-libs/aioredis/pull/802, [bpo32734](https://bugs.python.org/issue32734))
* now `aioredis_cluster.Cluster` do not acquire dedicate connection for every execute
* `aioredis_cluster` now requires `python>=3.6.5`

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* improve compatible with Python 3.8
* improve failover logic while command timed out
* read-only commands now retries if attempt_timeout is reached
* add required dependeny `async_timeout`
* `aioredis` dependency bound now is `aioredis >=1.1.0, <2.0.0`

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* add timeout for command execution (per execution try)
* add Cluster option `attempt_timeout` for configure command execution timeout, default timeout is 5 seconds
* Cluster.execute_pubsub() fixes

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* CHANGES fix

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* Cluster state auto reload
* new `state_reload_frequency` option to configure state reload frequency
* new `state_reload_interval` option to configure state auto reload interval
* `follow_cluster` option enable load cluster state from previous cluster state nodes
* establish connection only for master nodes after cluster state load
* change default commands_factory to aioredis_cluster.RedisCluster instead aioredis.Redis
* all cluster info commands always returns structs with str, not bytes
* `keys_master` and `all_masters` methods now try to ensure cluster state instead simply raise exception if connection lost to cluster node, for example
* `max_attempts` always defaults fix