walsender.c: Eliminate remaining uses of ThisTimeLineID.

walsender.c: Eliminate remaining uses of ThisTimeLineID.

IdentifySystem and StartReplication needs to know the current time
line ID, but it does not need to store the value into a global
variable, so don't do that.

logical_read_xlog_page is already relying on the timeline ID not
changing currently, so eliminat the use of the global variable
here, but also add some comments about the potential hazard if we
ever allow logical decoding on standbys.

To make all this work, GetStandbyFlushRecPtr now returns the TLI
via an out parameter.