txn_libmo: preserve confirmed_lsn after reading a PROMOTE

Previously we assumed that every PROMOTE request changes limbo owner,
and thus limbo should have confirmed_lsn = 0 after the request is
processed, because new confirmed lsn is yet unknown.

This is not true for PROMOTE requests coming in JOIN or saved in
snapshot: such requests don't change limbo owner: they are like
savepoints, they notify the instance of the current limbo state.

Such promotions may be detected by the rule
replica_id (old limbo owner) == origin_id (new limbo owner)

So, for the sake of correct split-brain detection, confirmed_lsn should
be nonzero after such promotions.

Part-of #5295

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