Change ThisTimeLineID from a global variable to a local variable.

StartupXLOG() still has ThisTimeLineID as a local variable, but the
remaining code in xlog.c now needs to the relevant TimeLineID by some
other means. Mostly, this means that we now pass it as a function
parameter to a bunch of functions where we didn't previously.
However, a few cases require special handling:

- In functions that might be called by outside callers who
  wouldn't necessarily know what timeline to specify, we get
  the timeline ID from shared memory. XLogCtl->ThisTimeLineID
  can be used in most cases since recovery is known to have
  completed by the time those functions are called.  In
  xlog_redo(), we can use XLogCtl->replayEndTLI.

- XLogFileClose() needs to know the TLI of the open logfile.
  Do that with a new global variable openLogTLI. While
  someone could argue that this is just trading one global
  variable for another, the new one has a far more narrow
  purposes and is referenced in just a few places.

- read_backup_label() now returns the TLI that it obtains
  by parsing the backup_label file. Previously, ReadRecord()
  could be called to parse the checkpoint record without
  ThisTimeLineID having been initialized. Now, the timeline
  is passed down, and I didn't want to pass an uninitialized
  variable; this change lets us avoid that. The old coding
  didn't seem to have any practical consequences that we need
  to worry about, but this is cleaner.

- In BootstrapXLOG(), it's just a constant.

Patch by me, reviewed and tested by Michael Paquier, Amul Sul, and
Álvaro Herrera.

Discussion: https://postgr.es/m/CA+TgmobfAAqhfWa1kaFBBFvX+5CjM=7TE=n4r4Q1o2bjbGYBpA@mail.gmail.com

src/include/access/xlog.h

@@ -261,7 +261,7 @@ extern void XLogFlush(XLogRecPtr RecPtr);
 extern bool XLogBackgroundFlush(void);
 extern bool XLogNeedsFlush(XLogRecPtr RecPtr);
 extern int	XLogFileInit(XLogSegNo segno, TimeLineID tli);
-extern int	XLogFileOpen(XLogSegNo segno);
+extern int	XLogFileOpen(XLogSegNo segno, TimeLineID tli);
 
 extern void CheckXLogRemoved(XLogSegNo segno, TimeLineID tli);
 extern XLogSegNo XLogGetLastRemovedSegno(void);