Get pubsub numsub working

This was a cherry-pick of 3ca471b

Get pubsub numsub working

Get pubsub numsub working

[Docs] Update README to include new dependency

[Docs] Update README to use emoji instead of markdown for docsgen

81 -- Add Configuration types for initialization

Change RedisConnection to end subscriptions when not allowed

Motivation:

When `RedisConnection.allowSubscriptions` is set to `false`, the connection could still be in a subscription state
leaving further commands to fail slowly from a full roundtrip to Redis, rather than succeeding as expected.

This changes the implementation so that it triggers a full unsubscribe from patterns and channels when set to `false`.

Modifications:

- Change: `RedisConnection.allowSubscriptions` to call `unsubscribe()` and `punsubscribe()` when set to `false`
- Change: `RedisPubSubHandler` to prefix storage of all dictionary keys to avoid name clashes between pattern and channel subscriptions

Result:

Developers should now have more deterministic and unsurprising behavior with PubSub
in regards to subscription management and connection state.

Add default buffer connectionRetryTimeout to avoid literal immediate …

…timeouts

Motivation:

When trying to allow users to configure the connection retry timeout offset,
not having a value provided (deadline of `now`) caused all attempts to use the pool to fail.

Modifications:

- Change: RedisConnectionPool to always have a timeout offset defined

Result:

If users don't specify any value, then the default of 60 seconds will be used.

If users specify "nil" (or `.none`) as the timeout, then a minimum of 10 milliseconds will be used to avoid immediate timeouts

Otherwise, use the user's specified `TimeAmount` as the offset of the timeout

Add configuration option for RedisConnectionPool lease timeout

Motivation:

With RedisConnectionPool a timeout is provided to prevent infinite loops of
retrying connections, but right now it is hardcoded to 60 seconds.

Users of downstream projects such as Vapor are noticing a "regression" of sorts, as previously
EventLoopFutures would fail immediately if a connection was not made available.

Modifications:

- Add: `connectionRetryTimeout` parameter to `RedisConnectionPool` initializer that still defaults to 60 seconds
- Change: RedisConnectionPool to use the new parameter if available to offset a deadline from "now"

Result:

Users can now configure the connection pool to fail immediately if connections are not available.

Add support for PubSub

Motivation:

One of the great features of Redis is being able to subscribe and receive messages published to specific channels
as a way of acting as a message queue for processing jobs.

PubSub requires a specific understanding of the connection model that can only be implemented directly in this library.

Modifications:

- Add: `RedisPubSubHandler` to sit in front of `RedisCommandHandler` to manage subscription callbacks and Redis registration
- Add: `publish` and the `pubsub` commands
- Add: `addPubSubHandler` extension to `NIO.Channel`
- Add: Type-safe String wrapper of `RedisChannelName` for PubSub methods
- Add: `pubsubSubscriptionNotFound` error case
- Add: `isSubscribed` property to `RedisConnection`
- Add: `availableConnectionCount` and `leasedConnectionCount` properties to `RedisConnectionPool`
- Add: Metrics for PubSub
- Add: `makeNewPool` factory method to `RedisConnectionPoolIntegrationTestCase`
- Change: `RedisClient` to require methods for PubSub management, as they are intrinsicly tied to the client's connection model
- Change: Parsing of `PING` response for handling special case in PubSub mode
- Rename: `ActiveConnectionGauge` to `RedisMetrics.IncrementalGauge`

Result:

Developers will now be able to use Redis in PubSub mode with both connections and pools.

This resolves #6