Rework XLogReader callback system

Code review for 0dc8ead463, prompted by a bug closed by 91c40548d5.

XLogReader's system for opening and closing segments had gotten too
complicated, with callbacks being passed at both the XLogReaderAllocate
level (read_page) as well as at the WALRead level (segment_open).  This
was confusing and hard to follow, so restructure things so that these
callbacks are passed together at XLogReaderAllocate time, and add
another callback to the set (segment_close) to make it a coherent whole.
Also, ensure XLogReaderState is an argument to all the callbacks, so
that they can grab at the ->private data if necessary.

Document the whole arrangement more clearly.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20200422175754.GA19858@alvherre.pgsql

src/backend/replication/slotfuncs.c

@@ -152,8 +152,10 @@ create_logical_replication_slot(char *name, char *plugin,
 	ctx = CreateInitDecodingContext(plugin, NIL,
 									false,	/* just catalogs is OK */
 									restart_lsn,
-									read_local_xlog_page, NULL, NULL,
-									NULL);
+									XL_ROUTINE(.page_read = read_local_xlog_page,
+											   .segment_open = wal_segment_open,
+											   .segment_close = wal_segment_close),
+									NULL, NULL, NULL);
 
 	/*
 	 * If caller needs us to determine the decoding start point, do so now.
@@ -464,7 +466,9 @@ pg_logical_replication_slot_advance(XLogRecPtr moveto)
 		ctx = CreateDecodingContext(InvalidXLogRecPtr,
 									NIL,
 									true,	/* fast_forward */
-									read_local_xlog_page,
+									XL_ROUTINE(.page_read = read_local_xlog_page,
+											   .segment_open = wal_segment_open,
+											   .segment_close = wal_segment_close),
 									NULL, NULL, NULL);
 
 		/*