Updated key WANObjectCache::delete() callers to avoid races

Updated key WANObjectCache::delete() callers to avoid races

* They now issue the delete() write before COMMIT of
  the relevant DB (or immediately if no trx is active)
* This can avoid some stale write race conditions
* Updated the WAN cache delete() docs

Change-Id: Id54887976051120b76528070d5f2ceb357d57897