SPY-160: Avoid ConcurrentModificationException with the retry queue

Motivation
----------
ConcurrentModificationException will show up since the code can recursively
modify itself.

Modifications
-------------
Copy the original list into a temporary new one to avoid the exception.

Result
------
Redistribute on the retry list will work as expected, without an
exception.

Change-Id: Ieff1a87136bef38280b7f139192419073d6a878a
Reviewed-on: http://review.couchbase.org/34977
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>