fix(execution): fan-out can start an account after the run deadline #1583

In internal/execution/fanout.go (~lines 89-93), the deadline check and the account start aren't atomic, so an account can still start after the run deadline. A fan-out run with a slow dependency (for example a hung mail server) can then go past the 300s Lambda timeout. #1578's PR body says accounts are skipped once time is up, but that isn't guaranteed.

Fix: re-check the context deadline at the point each account's work starts, and add a test where the deadline passes between scheduling and start.

Found in review of #1578. It predates #1578 and isn't caused by it.