objectcache: add mcrouter support to WANObjectCache

objectcache: add mcrouter support to WANObjectCache

* Update documentation about relay methods.
* Change interim key set() to add() to avoid broadcasting it.
* Remove the behavior of doing purges synchronously in the
  local DC first before relay. In both the event relayer and
  mcrouter case, they will be asynchronous. It was hardly
  even possible to use such behavior since loads come from
  slave DBs, which do not see changes right after COMMIT.

Bug: T97562
Change-Id: I7759c82ae6e1b72fc227882a99c9a712a46374f6