PBM-1311: PBM fails to do PITR slicing when RS doesn't have a majority

[boris-ilijic]

Fix for https://perconadev.atlassian.net/browse/PBM-1311

When doing PITR slicing PBM relies on majorityOpTime to define slicing intervals:

db.isMaster()
{
  ...
  lastWrite: {
    opTime: { ts: Timestamp({ t: 1715079500, i: 1 }), t: Long('1') },
    lastWriteDate: ISODate('2024-05-07T10:58:20.000Z'),
    majorityOpTime: { ts: Timestamp({ t: 1715079500, i: 1 }), t: Long('1') },
    majorityWriteDate: ISODate('2024-05-07T10:58:20.000Z')
  },
 ...

When cluster doesn't have majority majorityOpTime is stale, so it contains last value when majority was present.

This PR fixes that using approach: when write concern is below majority, PBM uses opTime instead of majorityOpTime to define PITR slicing intervals.