Follow up work for SERVER-63789

2 followup works:
1) Currently, we call thread join() within the mutex lock which can cause deadlocks (_thread waiting for the mutex lock, and reset() thread takes the mutex lock and wait for the _thread to join). We should modify the code such that all thread joins happen outside of the mutex lock.

2) Currently opCtx interruption logic in the tenantFileImporter Service is broken.  We need a reliable way for the interrupt thread to interrupt the spawned thread's opCtx.