Clean up BagOStuff::get() interface

Clean up BagOStuff::get() interface

* Callers of get() no longer have to contend with
  the annoying $casToken parameter, which is there
  but totally unusable to non-BagOStuff code.
* The default get() now delegates to doGet(),
  which callers must implement instead. They can
  ignore the overhead of generating $casToken if
  they do not implement cas(), which applies to
  callers that use the stock merge(). If cas() is
  used for merge(), then getWithToken() must be
  implemented.
* Also add BagOStuff::READ_LATEST to mergeViaCas()
  for sanity, as that missing before.
  Likewise with mergeViaLock().

Change-Id: I4efce6a9ab4b1eadd2f161dff641004a7239c516