[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 `UpdateInfo`s.

See [ENG-4563](https://linear.app/comm/issue/ENG-4563/doubled-messagenotification) 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

keyserver/src/creators/message-creator.js

@@ -185,7 +185,7 @@ async function createMessages(
       creation, target_message)
     VALUES ${messageInsertRows}
   `;
-  const messageInsertPromise = dbQuery(messageInsertQuery);
+  await dbQuery(messageInsertQuery);
 
   const postMessageSendPromise = postMessageSend(
     viewer,
@@ -204,7 +204,6 @@ async function createMessages(
   }
 
   await Promise.all([
-    messageInsertPromise,
     updateRepliesCount(threadsToMessageIndices, newMessageDatas),
     viewer.isScriptViewer ? postMessageSendPromise : undefined,
   ]);