Keep trying to claim in stress tests

Don't give up trying to claim new work if claim returns nil. Only give up once
a deadline has passed.
Retry claim in election-handoff-test
Retry claims in node test
Sleep a little between retries
Increase deadlines and sleep time
Fix a couple more times
Merge pull request #92 from eric/keep-trying-to-claim-in-stress-test

Keep trying to claim in tests