As of SERVER-40080 here the "wall" field of oplog entries no longer became optional like it is in the IDL. If we used the IDL for parsing the oplog entry there, then this backwards breaking change wouldn't have happened since the IDL considers it optional. This actually should be safe since we've been adding 'wall' since 3.6 and it's about time we made it mandatory, but we should do that in the IDL, not in one particular place where we parse the oplog entry individually.

Specifically, we should use the OplogEntry class to parse oplog entries everywhere.