Follow-on work to SERVER-110493.

This code properly belongs in oplog_truncation.cpp, and its unit test belongs in oplog_truncation_test.cpp.  Dependency management means that to do this we probably also have to change the return value of newestExpiredRecord() to return a RecordId (which would otherwise be a separate task).

This task also covers moving the unit test out of collection_truncate_markers_test.cpp.