config: Split "Worker Processes" out of "Asynchronous Behavior"

config: Rename "Asynchronous Behavior" to "I/O"

Change shutdown sequence to terminate checkpointer last

postmaster: Adjust which processes we expect to have exited

postmaster: Commonalize FatalError paths

postmaster: Don't repeatedly transition to crashing state

postmaster: Move code to switch into FatalError state into function