Sleep between object ZBF process forks

We've found that, on fresh systems where the ZBF run completes
almost instantaneously, the Swift logs (syslog by default) get
filled-up with object ZBF scan start and completion messages.
This patch calls self._sleep() between ZBF scan runs to cut-down
on these messages and the related unnecessary ZBF scan runs
without impacting the integrity of the object auditing process.

Change-Id: I057c5ca235467cfa115a7a3d44e21c350900059a
Merge "Sleep between object ZBF process forks"