8.8 - subkey (hash field) notifications

[mgravell]

Server 8.8 has new subkey (hash field) notifications; we:

• add new RedisChannel APIs for building channels
• enhance KeyNotification to understand the new types
• change KeyNotification to store the kind as an enum, rather than constantly checking patterns
• add a new API to iterate the sub-key(s) associated with an event
• add unit tests
• include the new API in documentation
• release note
• rev CI to a version with the feature
• add integration tests