Bug 4190: assertion 'hash_remove_link' from Auth::User::cacheCleanup

Bug 4190: assertion 'hash_remove_link' from Auth::User::cacheCleanup

The hash_link based cache depends on raw-ptr key comparisons to store
hash entries. This does not work at all well with SBuf as the key,
since the backing MemBlob behind SBuf can change its memory location.

* replace the implementation of User credentials caching with an STL
  based container class that can handle SBuf.

* revert the global Auth::User cache design to per-scheme caches
  which get combined only when reporting statistics.

* add a RunnersRegistry helper class to control Squid startup,
  reconfigure, and shutdown events activity in regards to the caches.

* suppress useless cache garbage collection events when auth has no
  credentials to cleanup.

* make the cache key dynamic at the caller codes discretion.