Refactor Logging implementation

Refactor Logging implementation

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 (closed) and #74 (closed)