Implement wp_cache_flush_group()

Per https://core.trac.wordpress.org/ticket/4476

This would require adding an additional flush group on a per-cache-group basis.

Falling back to `wp_cache_flush()` would cause issues if a plugin on the main site attempted to flush a specific cache group, as it'd rotate every cache.

Start with the linked WordPress Trac ticket 4476 and the wp_cache_flush_group() and wp_cache_flush() entry points named in the issue. Trace how cache groups are handled, then verify that flushing one group does not rotate or flush every cache group.

Written by the indexing model from the issue text.