Add SessionInfo force-use flag

Add SessionInfo force-use flag

A provider that uses SessionProvider::hashToSessionId() will likely have
issues if something such as a call to $user->setToken() causes
SessionManager::loadSessionInfoFromStore() to fail, since the provider
can't just arbitrarily change the session ID it returns.

The two solutions to this problem are:
* Somehow include everything that could cause loadSessionInfoFromStore
  to fail in the data hashed by hashToSessionId.
* Flag the SessionInfo so that, if stored data and the SessionInfo
  conflict, it should delete the stored data instead of discarding the
  SessionInfo.

Since the second is less complexity overall due to the lack of need to
define "everything", this patch takes that approach.

Change-Id: I8c6fab2ec295e71242bbcb19d0ee5ade6bd655df