The config.system.sessions collection is synced with the in-memory cache periodically. Defunct sessions are removed using multi-deletes (mutliple deletes in a single oplog entry) but new/updated records are still replicated individually.

The goal of this ticket is to better understand this replicated behavior with a js test in the jstests/noPassthrough directory. This will facilitate improvements to the upsert logic used in the session management code.