We need to provide a hook for transaction expiration which is invoked on all standalones and primaries to alert the retryable write subsystem when sessions have expired.

The lifetime and logic here seems identical to the rules for cursors (The cursor manager tells us what sessions are used by cursors, then we tell the cursor manager to drop the cursors attached to those sessions). We can replace the first step with a scan of the transaction table and the second with invocation of a callback that retryable writes can fill in (either via a callback injection or having them implement a service liason method).