xlog.c: Use XLogCtl->ThisTimeLineID in various places.

xlog.c: Use XLogCtl->ThisTimeLineID in various places.

Where appropriate, instead of using ThisTimeLineID, use the value
from shared memory instead, to reduce dependencies on the global
variable. This is only safe after recovery is complete, because
prior to that, the global variable and the shared state don't
match.

This allows removal of some ugly logic in CreatRestartPoint() to
temporarily set ThisTimeLineID for purposes of tricking
RemoveOldXlogFiles and PreallocXlogFiles into doing the right thing,
and then clearing it again afterwards.