Adding speed limit options for DB auditor

Fix bug 1129760

Without speed limit, DB auditor will likely consume high CPU% on
storage node. That will highly impact the cluster's performance.

This patch adds two options for account/container auditor:
 - containers_per_second: Maximum containers audited per second
 - accounts_per_second: Maximum accounts audited per second

DocImpact

Change-Id: I9faa506438185a83ca77db4906969328624d015f
Merge "Adding speed limit options for DB auditor"