Slight modification to the object updater sweep code

In preparation for proposing the meat of storage policies to
master, we're taking small and harmless portions that may not
always appear to add value now but they will when storage
policies come in and geting them in now will minimize the size
of upcoming merges.

In this change, we're changing the name of ASYNCDIR to
ASYNCDIR_BASE as policies will have multiple async directory
names (one per policy).  In the object updater rather than just
looking for the one name 'asyncdir' we set the stage for looping
on all subdirs looking for things that start with ASYNCDIR_BASE
and moving on to the next dir if its not a match.  Introduction
of this code to master now adds no new functionality; one small
block of code will be merged later to actually match on names
other than ASYNCDIR_BASE and do the right thing.

Change-Id: I655d2f2fdadc050ad95af5d1237ebe828b06701a
Implements: blueprint storage-policies