This ticket is split out from
SERVER-34598. SERVER-40080 will now be used to track the patch to include wall clock times corresponding to the lastApplied and lastDurable optimes only. The tasks required for SERVER-40080 and SERVER-40078 are different, and the lastApplied/lastDurable changes are sufficiently complex on their own.
The current plan for implementing this follows. Since
SERVER-40080 allows each member of a replica set to track its own last applied and last durable wall clock times, they can communicate this information to the primary similarly to how they communicate their OpTime information (i.e., through replSetUpdatePosition, heartbeats, etc.) so the majority committed wall clock time can be calculated similarly to the way the majority committed OpTime is calculated.