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