Shutdown REST api correctly

[Babicaa]

When the writer finishes, it might kill the REST Api in the middle of a request - it is a bit hard to reproduce, because it needs to happen on a very precise timing.

It was observed while running multiple scans in rapid succession at SF.

Need to change the current code not to call exit from the writing thread, but returning control to the REST thread and stopping the entire process.

Example:
https://github.com/ipkn/crow/blob/05d02fea6e2cf5d2603655419338995549993a8a/tests/unittest.cpp