Especially useful when interacting with getWithSetCallback of
HashBagOStuff where a $exptime is required before the callback.
In set() it is typically left out and thus not as counter-intuitive.
Change-Id: Ic2f7adda3c00cefe701d77bea91a7e8e77ef1439
Also consistently use self:: instead of BagOStuff:: for constants
referenced within the BagOStuff class.
Change-Id: I20fde9fa5cddcc9e92fa6a02b05dc7effa846742