Add a new field to oplog entries, "tid", that stores the tenantId. This field should be part of the DurableReplOperation struct, and should be boost::optional.

i.e.:

{
    op: 'i',
    ns: "<database>.<collection>", ui: <existingUUID>,
    tid: "<tenant>",
    o: { ... }, ...
}

Include this as a parameter in DurableOplogEntry::DurableOplogEntry and makeOplogEntryDoc.