Allow discovery of whether a dynamic background worker is running.

Using the infrastructure provided by this patch, it's possible either
to wait for the startup of a dynamically-registered background worker,
or to poll the status of such a worker without waiting.  In either
case, the current PID of the worker process can also be obtained.
As usual, worker_spi is updated to demonstrate the new functionality.

Patch by me.  Review by Andres Freund.

src/backend/commands/async.c

@@ -207,8 +207,6 @@ typedef struct QueueBackendStatus
 	QueuePosition pos;			/* backend has read queue up to here */
 } QueueBackendStatus;
 
-#define InvalidPid				(-1)
-
 /*
  * Shared memory state for LISTEN/NOTIFY (excluding its SLRU stuff)
  *