JCBC-309: Catch ConcurrentModificationException in IO Thread.

This changeset adds the check to the overriden method, it has
also been incorporated in spy directly.

Change-Id: Ic5d665b610fbf228408b798bf347c8835371efa7
Reviewed-on: http://review.couchbase.org/26465
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>