Turn PGBE_ACTIVITY_SIZE into a GUC variable, track_activity_query_size.

As the buffer could now be a lot larger than before, and copying it could
thus be a lot more expensive than before, use strcpy instead of memcpy to
copy the query string, as was already suggested in comments. Also, only copy
the PgBackendStatus struct and string if the slot is in use.

Patch by Thomas Lee, with some changes by me.

src/backend/utils/misc/postgresql.conf.sample

@@ -364,6 +364,7 @@
 #track_activities = on
 #track_counts = on
 #track_functions = none			# none, pl, all
+#track_activity_query_size = 1024
 #update_process_title = on