[keyserver] await INSERT INTO messages before calling postMessageSend

Summary:
We want to check if there is a collision on the INSERT before calling postMessageSend in order to avoid creating duplicate notifs or UpdateInfos.

See ENG-4563 for more details.

Test Plan: I tested message creation to make sure it still works

Reviewers: kamil, michal, atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D8700