objectcache: Deprecate ObjectCache::getWANInstance

objectcache: Deprecate ObjectCache::getWANInstance

This is not used anywhere. Once removed we can remove a bunch
more protected/private methods in this class that are all only
here still due to this one public method.

It was obsolete when the creation of WANObjectCache was
moved to ServiceWiring. In theory multiple instances can be
created but in the future that should be done by either constructing
the WANObjectCache directly where needed, or by implementing
it as a custom service.

Also deprecate ObjectCache::newWANCacheFromParams. This should
have been `@internal`, but oh well. Deprecate as well since
only needed by getWANInstance(). No longer used internally
either as of I118b6f01e4940.

Change-Id: Icc500e76c860c35ff40fdee47142c63091829dfe
(cherry picked from commit b10b56dc6923062314f136945da255a24e52a585)