Refactor WAL file-reading code into WALRead()

XLogReader, walsender and pg_waldump all had their own routines to read
data from WAL files to memory, with slightly different approaches
according to the particular conditions of each environment.  There's a
lot of commonality, so we can refactor that into a single routine
WALRead in XLogReader, and move the differences to a separate (simpler)
callback that just opens the next WAL-segment.  This results in a
clearer (ahem) code flow.

The error reporting needs are covered by filling in a new error-info
struct, WALReadError, and it's the caller's responsibility to act on it.
The backend has WALReadRaiseError() to do so.

We no longer ever need to seek in this interface; switch to using
pg_pread().

Author: Antonin Houska, with contributions from Álvaro Herrera
Reviewed-by: Michaël Paquier, Kyotaro Horiguchi
Discussion: https://postgr.es/m/14984.1554998742@spoje.net

src/include/access/xlogutils.h

@@ -54,4 +54,6 @@ extern int	read_local_xlog_page(XLogReaderState *state,
 extern void XLogReadDetermineTimeline(XLogReaderState *state,
 									  XLogRecPtr wantPage, uint32 wantLength);
 
+extern void WALReadRaiseError(WALReadError *errinfo);
+
 #endif