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/utils/cache/inval.c

@@ -1453,8 +1453,8 @@ CacheInvalidateRelcacheByRelid(Oid relid)
  * replaying WAL as well as when creating it.
  *
  * Note: In order to avoid bloating SharedInvalidationMessage, we store only
- * three bytes of the backend ID using what would otherwise be padding space.
- * Thus, the maximum possible backend ID is 2^23-1.
+ * three bytes of the ProcNumber using what would otherwise be padding space.
+ * Thus, the maximum possible ProcNumber is 2^23-1.
  */
 void
 CacheInvalidateSmgr(RelFileLocatorBackend rlocator)