Upgrade redis-py to 4.0.0 (brainix#504)

Make RedisList/RedisDeque .insert() use pipelines (brainix#484)

* Make RedisList/RedisDeque .insert() use pipelines

This avoids race conditions.

* Fix inserting into RedisList w/ repeated elements

* Bump version number

* Increase unit test coverage

* Use pipeline to get len in RedisDeque.__extend()

* Refactor RedisList.insert() and RedisDeque.insert()

Don't repeat myself.

* Delete redundant unit tests

Use BailOutExecutor in more places (brainix#482)

This makes `Redlock.locked()` and the `NextId.__current_id` getter more
efficient for several masters.

Fix RedisList.__eq__() (brainix#476)

* Fix RedisList.__eq__()

1. If `self` is `other`, return `True`
2. If `self` and `other` live on the same Redis database with the same
   key, return `True`
3. If `self` and `other` are of different lengths, return `False`
4. If `self` and `other` are ordered and have the same elements, return
   `True`
5. Otherwise, return `False`

* Improve docstrings and comments

* Make RedisList.__eq__() performant by using LRANGE

Flesh out RedisSet methods (brainix#470)

* Flesh out RedisSet methods

Implement `RedisSet.issubset()`, `RedisSet.issuperset()`, and
`RedisSet.intersection()`.  Refactor `RedisSet.isdisjoint()` to use
`RedisSet.intersection()`.

* Bump version number

Delete the Redis.lolwut() monkey patch (brainix#464)

* Delete the Redis.lolwut() monkey patch

I got `Redis.lolwut()` merged upstream!
redis/redis-py#1568

* Bump version number

In NextId.reset(), reset all Redis masters (brainix#463)

* In NextId.reset(), reset all Redis masters

Previously, I'd modified the `NextId.__current_id` getter to get the ID
from `n // 2 + 1` Redis masters, then to return the highest of those
IDs.

This means that `NextId.reset()` must delete the Redis key from all
masters.

* Upgrade requirements

Make consensus-based read ops more efficient (brainix#462)

On read operations, once we've achieved quorum and read the value that
we need, cancel pending in-flight futures.

Implement NextId.reset() (brainix#459)

* Implement NextId.reset()

`NextId.reset()` deletes the `NextId` object's key from the Redis
masters, in effect resetting the current ID to 0.

* Reorder unit tests to reflect order of methods

* Unit test NextId.reset() quorum exceptions

* Rewrite NextIdTests.test_reset() to tell a story

* Bump version number

* Dedent quorum exceptions out of executor block

Make Pottery work for both bytes and str responses (brainix#458)

* Make Pottery work for both bytes and str responses

By default, the Redis client returns `bytes` responses.  However, the
Redis client can be configured to return `str` responses.  Make Pottery
work for either type of responses.

* Make whitespace consistent