more informative error message for bad serialization
check that the unpacked test object is correct
test invalid Session pack and unpack
Merge pull request #4239 from minrk/better-errors-for-aaron

more informative error message for bad serialization
change default timeout to 10s from infinite in parallel tests

avoids hangs with apply_sync in tests
adjust Scheduler timeout logic

Timeout starts when task submission is attempted.
That is, it starts *each time* in resubmit / retry cases.

Avoids issues of timeout firing only once while a task is pending,
when it might come back and be retried after the timeout has fired.
test very short timeouts

which should be guaranteed to fire while the task is pending