SERVER-137224 can be improved by 

1. Parsing the first op exactly once
2. Parsing the checkpoint into a usable data structure within isCheckpointApplyOpsEntry()

Combine isCheckpointApplyOpsEntry() and parseCheckpoint() into a single function called tryParseCheckpoint():

struct ParsedCheckpoint {
    CheckpointContents checkpoint;
    Timestamp validAsOf;
}

boost::optional<ParsedCheckpoint> tryParseCheckpoint(const repl::OplogEntry& entry);

Record the checkpoint if it was parseable. Otherwise don't. This interacts with SERVER-136501, so do that first.