SPY-112: Throw CancellationException when future is cancelled.

This changeset throws the semantically more correct
CancellationException when a operation future is cancelled. Old
code that catches RuntimeExceptions will still work, but it allows
for more flexible catching on the application level.

Change-Id: I476f06e8989f06423be6e186c6565179e270df13
Reviewed-on: http://review.couchbase.org/24654
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>