Replace BackendIds with 0-based ProcNumbers

Now that BackendId was just another index into the proc array, it was
redundant with the 0-based proc numbers used in other places. Replace
all usage of backend IDs with proc numbers.

The only place where the term "backend id" remains is in a few pgstat
functions that expose backend IDs at the SQL level. Those IDs are now
in fact 0-based ProcNumbers too, but the documentation still calls
them "backend ids". That term still seems appropriate to describe what
the numbers are, so I let it be.

One user-visible effect is that pg_temp_0 is now a valid temp schema
name, for backend with ProcNumber 0.

Reviewed-by: Andres Freund
Discussion: https://www.postgresql.org/message-id/8171f1aa-496f-46a6-afc3-c46fe7a9b407@iki.fi

src/backend/backup/basebackup_incremental.c

@@ -672,7 +672,7 @@ GetIncrementalFilePath(Oid dboid, Oid spcoid, RelFileNumber relfilenumber,
 	char	   *lastslash;
 	char	   *ipath;
 
-	path = GetRelationPath(dboid, spcoid, relfilenumber, InvalidBackendId,
+	path = GetRelationPath(dboid, spcoid, relfilenumber, INVALID_PROC_NUMBER,
 						   forknum);
 
 	lastslash = strrchr(path, '/');