Revert "remove deferral.sleep's canceller, as it wasn't used anywhere and caused incompatibilities with older Twisted versions"

This reverts commit b3a4925e7eb1e99ce6e304fc10b60dfc16fb5ee7.
replace task.LoopingCall's with deferral.RobustLoopingCall that catches errors and continues
Merge pull request #2 from forrestv/master

update to current forrestv p2poool