Support staggering nodes of read-only swap

After fetching new read-only data files, optionally stagger the swaps
performed on each of the nodes within the cluster. A new --stagger
option has been added to the swap command for this purpose.
Update release notes for release 1.6.0-ta.1
Merge pull request #1 from mkralka/release-1.6.0-ta.1

Support staggering nodes of read-only swap