Relay follow-ups and ideas

A first version of Relay support is being added in 📌 Explicitly support Relay (drop-in replacement for PhpRedis) Fixed , there have been some ideas and things that we noticed that could be improved, both in this module and in Relay itself.

• Support for caching mget() nil responses, this is heavily used by the cache tag checksum implementation. As a first improvement, single cache tag lookups were changed to using a single get() lookup
• Use allow patterns for in-memory caching instead of ignore patterns, might allow to cache the last-delete flag as well for all bins
• Use the provided serialization (igbinary) and compresssion API, unclear if the available methods are meant to be a (stable) API
• Not relay specific: Improve documentation and recommendations around persistent connections
• Make use of addIgnorePatterns(), which requires version 0.6.0, the tests currently install an older version due to outdated metadata.