[keyserver] Log on keyserver when INITIAL_NOTIFICATIONS_ENCRYPTED_MESSAGE is sent to the client

Summary: https://linear.app/comm/issue/ENG-9932/log-on-keyserver-when-initial-notifications-encrypted-message-is-sent

Test Plan: Flow and put the log somewhere else and check if it logs (I don't know how to trigger olmNotificationsSessionMissing)

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13951