In https://github.com/10gen/mongo/pull/33226 we needed the functionality "kill the oldest session" but piecing this together from the existing API was suboptimal, because of the way that class manages its locks through the API.

Some options to improve it:

• Add a "reduce" kind of API, SessionCatalog only provides "forEach" kind of functionality, but we don't know what the oldest is until after we've scanned them all, and we had to yield locks in between figuring the oldest and killing it.
• make kill return optional (or StatusWith) rather than except. This at least makes it clear to callers that the session you ask for might not be there any more.
• Facilitate manual locking (the "WithLock" pattern), tbh i don't like exposing locks over APIs given the above two options...