Use PRS in Vtorc for electing new primary

[GuptaManan100]

Description

This PR changes the implementation of electNewPrimary to use PlannedReparentShard instead of having its own codepath. It also changes some checks in PlannedReparentShard to allow calling it with no new primary specified even for initialisation. No new end to end test has been added for this since there is already extensive coverage in the vtorc's tests which rely on electNewPrimary (PlannedReparentShard) to initialise the cluster.

Related Issue(s)

• RFC - Durability and consensus in Vtorc #8975

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[GuptaManan100]

The implementation of the AtLeast method on Position has also been fixed after I found the bug via a unit test that I was adding that compared a nil position with a non-nil one. In this case, we should say that the non-nil position is at least as advanced as the nil one. On further investigation it turned out that the underlying function call to Contains does not handle nil case correctly as changed in 031e3a3. So nils have to be handled at a level above that function call.