Made User generally use DB_SLAVE by default

Made User generally use DB_SLAVE by default

* By default, users will load from the slave unless
  the thread did a recent DB write. This is to handle
  changes within a request.
* ChronologyProtector should avoid staleness in common
  cases, and the CAS check on user_touched is a final
  barrier to block stale user object updates.
* Note that passwords are not cached, so they hit the
  DB when ever needed. Passwords now load from slaves
  when possible, instead of the master.
* This should get the code closer to handling user
  login and logged in users when the master is down.
* Fixed loadFromId() when READ_LOCKING is used.
* Also addressed TODO comment in load().

Bug: T92357
Change-Id: I0a8bdab720c19fe3fc2381799ae2e90ff09bb4cf