txn_limbo: track CONFIRM lsn on replicas

limbo->confirmed_lsn was only filled on limbo owner in
txn_limbo_write_confirm. Replicas and recovering limbo owner need to track
it as well to correctly detect split-brains based on confirmed_lsn.

So update confirmed_lsn in txn_limbo_read_confirm.

Part-of #5295

NO_DOC=internal change
NO_TEST=tested in future commits
NO_CHANGELOG=internal change