Motivation:

The original implementation of Logging was done in more haste than should have been, without proper attention given to the semantic requirements.

As the Swift ecosystem has matured a bit, lessons have been learned on handling metadata and passing of external context into internal subcomponents.

A mixture of the "protocol-based context passing" and "explicit context passing" patterns have been adopted.

Both patterns are more fully described in the Swift forum discussion: https://forums.swift.org/t/the-context-passing-problem/39162

Modifications:

- Add: `RedisLogging` namespace with references to static keys and labels that are used for Logging throughout the library
- Add: `Logger` static computed properties to access the Logger prototypes used in connection and connection pools
- Add: `RedisClientWithUserContext` protocol and `UserContextRedisClient` types to assist with wrapping client types for custom logger contexts
- Remove: `logger` property from `RedisClient` requirements
- Change: Many log statements to have higher or lower log levels for their appropriate context
- Change: `RedisConnection` and `RedisConnectionPool` to conform to `RedisClientWithUserContext`
- Change: `logging(to:)` protocol requirement to return a `RedisClient` existential
- Change: ConnectionPool to explicitly pass a logger instance around for pooling methods

Result:

Logging in RediStack will now have a stronger contract of where and how logs will be generated and which context will be used.

Fixes #79 and #74