Consider a secondary Node 0 syncing from a member, Node 1.

When Node 0 executes TopologyCoordinator::shouldChangeSyncSource, it checks whether Node 1 thinks the primary has changed. Node 0 uses unsafe logic for this purpose: it checks the OplogQueryMetadata.currentPrimaryIndex it received from Node 1, but Node 0 doesn't know if it has the same config as Node 1 or not, so it doesn't know what member currentPrimaryIndex refers to.

Fortunately, all that Node 0 needs to know when it executes TopologyCoordinator::shouldChangeSyncSource is whether Node 1 itself is the primary. Add an isPrimary field to ReplSetMetadata, and deprecate currentPrimaryIndex.

Before closing this ticket, open another ticket to remove OplogQueryMetadata.currentPrimaryIndex in the next release.

(Discovered during the SERVER-46667 investigation.)