R1:78d1b62bb8e9

Streamline handling of Futures and PIDs in daemons

Summary:
Ref T13555. Currently, the daemon future may resolve into a failure state immediately inside "start()", and not have a valid PID when we read it.

Instead, read PIDs from the current active future in all cases, using "hasPID()" to test for the presence of a valid PID.

Since we don't query the PID immediately, we no longer need to explicitly start the future.

Also fix an issue where the same future could be added to the overseer pool more than…