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

Initial sync 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).

This ticket is to investigate if it's possible that a node undergoing initial sync observes an upgradeDowngradeTimeseries oplog entry and how to react to it.

We should also investigate if there's any possible problem with other oplog application modes, e.g. recovery from unstable checkpoint.