Sessions spawned to run transactions for retryable writes encode the write's txnNumber in their session id. When a new retryable write / transaction for that same session arrives with a higher txnNumber, previous sessions can no longer be used, so we are free to reap their config.transactions entries. This should be done more eagerly than waiting for the config.system.sessions entry to expire, since a long running session may run many retryable writes that could each require a transaction and create a config.transactions entry.