xF2 Add-on Redis Cache By Xon 2.17.2

• Workaround for persistent connections and XF2.1 page cache feature causing login failures when using multiple databases in the same redis instance.
• Track latest upstream Credis library

• Enable LUA by default (all Redis versions since v2.8 have supported LUA scripting)
• Multi-DB support when counting keys in Redis instamce
• Support being a drop in replacement for the stock Redis provider
  • Alias the config key "host" to "server"

Thanks to @DragonByte Tech for contributions

• Update layout of master Redis server to match server environment report
• Include free/peak/max memory usage in stats output

New option to default disable XenForo's individual templating css caching.

• XenForo2 provides per-template css caching which is vulnerable to "thundering herd" cache slamming by introducing unexpected write load which is vulnerable to MySQL locking via insert ... on duplicate key update.
• This is only an issue for sites which experience large numbers of concurrent users
• The full CSS output is still cached by whatever caching layer is defined, which saves vastly more time than individual template caching

• Support zstd compression
• New retry_reads_on_master option, when a read to the slave-redis instance fails (ie due to the redis instance still loading data), reads will fail-back to the master instance.
• Confirm php 7.3 compatibility for pure-php redis connector
• Compat bugfix for phpredis-v3 for exists() function. phpredis-v3 returns a bool, credis/phpredis-v4 return an int, this ensures all backend redis connectors return the same type for the exists function.

• Make AdminCP redis info block collapsable

• Ensure cached css entries expire after ~1 hour like stock XenForo caching does.
  • On install, all existing css entries will be purged from the configured redis instance

• Tweak execution order of class extensions/template modifications/etc to provide predictable ordering
• Add LICENCE and README.MD to redistributable.

If decompression of fetched content fails, just fail the cache get request. This should be a safe trade-off as if the key returns nothing XF stack should regenerate it.

• Makes changing compression type a cache flush rather than rendering the site useless until Redis is manually flushed.

Support configuration-less usage to match built-in Redis provider.

• If no server has been configured try localhost