refactor(Imap_Client): deprecate Cache_Backend_Hashtable in favor of Cache_Backend_Cache

[ralflang]

Horde_Imap_Client_Cache_Backend_Hashtable marked @deprecated. The per-UID HashTable storage strategy was justified for Memcache 1.0 with poor multi-get; modern Redis with MGET/pipelining handles the sliced strategy of Backend_Cache equally well, so one code path covers both.

UPGRADING.rst documents the migration path. IMP's Imap/Cache/Wrapper.php falls 'hashtable' driver configurations through to 'cache' transparently, so existing deployments don't need to update their cache.driver config.

Refs horde/Core#131

[what-the-diff]

PR Summary

• Improvement to Upgrade Guidelines
  We've made the upgrading process more comprehensive by adding instructions on how to smoothly transition to version 3.0.0. This includes a notice about the outdated Horde_Imap_Client_Cache_Backend_Hashtable and steps to switch to using the Backend_Cache.

• Deprecation Notice for Hashtable.php
  In our continual efforts to keep our codebase up-to-date and efficient, we've marked Horde_Imap_Client_Cache_Backend_Hashtable.php as deprecated. This means it will no longer be maintained and it is recommended to switch to Horde_Imap_Client_Cache_Backend_Cache. The reasons for this decision have also been provided in the comment section of the file for further understanding.