Report the WT all_durable timestamp in serverStatus

It may be helpful to report the "no holes" point for debug-ability. For example for flow control, where replication could be held up by an oplog hole or because of throttling.

Note: replSetGetStatus already returns the last durable optime and walltime. Primaries periodically update last durable with the WT all_durable point, via the journal flushing logic. Secondaries, however, do not periodically fetch the WT all_durable timestamp. I'm envisioning either: the serverStatus section reports differently for primary mode vs secondary mode, taking advantage of the journal flushing lookups; or both primary and secondary mode fetch from the storage engine directly (not sure whether there's a perf consequence to doing so).