Fix mem backend to honor various settings correctly.

@angrycub is testing.

test grid spreadsheet (basho-only):
https://docs.google.com/a/basho.com/spreadsheet/ccc?key=0AkSRjImmlw_HdG5TeVZ2dFRtUFpiZjI2YThQRHFGRWc#gid=0

Without this patch, the memory backend can 'run out of space' in that all of its memory is marked as used, while there is plenty of memory available.  Expiry can do similar things.  This patch makes sure that the state tuple is preserved through all vnode actions, as well.

This should be ported to 2.0 prior to the RC.  

cc @reiddraper re #717