Reduce impact of btree page reuse on Hot Standby by fixing off-by-1 error.

Reduce impact of btree page reuse on Hot Standby by fixing off-by-1 error.
WAL records of type XLOG_BTREE_REUSE_PAGE were generated using a
latestRemovedXid one higher than actually needed because xid used was
page opaque->btpo.xact rather than an actually removed xid.
Noticed on an otherwise quiet system by Noah Misch.

Noah Misch and Simon Riggs