objectcache: improve BagOStuff arithmetic method signatures

objectcache: improve BagOStuff arithmetic method signatures

Make the default $init value for incrWithInit() be $value.
This is far less suprising and also makes the operation
easier to replicate without conflicts.

Make decr() definitions more explicit since various cache
drivers do not handle negative incr() values (e.g. memcached).

Change-Id: I2b8d642656cc91c841abbd7a55d97eba101b027a