SERVER-114505 introduced a new type of oplog 'c' entry upgradeDowngradeViewlessTimeseries that is generated when converting from/to viewless timeseries on FCV upgrade/downgrade.

Initial sync and unstable recovery does not apply oplog entries from a consistent snapshot, which means that the oplog application code has to tolerate working in this inconsistent state.

(At the same time initial sync is incompatible with FCV changes - see the initial_sync_fcv.js test - so we may not need to support it).

This ticket is to investigate how to deal with upgradeDowngradeViewlessTimeseries oplog entries being applied in those two modes.