Consolidate tracker file I/O
Consolidate tracker file I/O

There’s a minor issue with the current implementation, in that if the
machine’s memory usage gets completely swallowed up, the open file seems 
to have a pretty high probability of getting wiped out, which causes
subsequent worker runs to run against the _entire_ Salesforce dataset.

It’s safer to close the file descriptor between actual file writes to 
avoid these sorts of shenanigans.
tablexi/restforce-db,tablexi/restforce-db,tablexi/restforce-db