Denormalize buffer operations (#9026)

This should significantly reduce database load on redeploy.

Co-Authored-By: Max <max@zed.dev>
Co-Authored-By: Nathan <nathan@zed.dev>

Release Notes:

- Reduced likelihood of being disconnected during deploys

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>

crates/collab/migrations.sqlite/20221109000000_test_schema.sql

@@ -248,7 +248,10 @@ CREATE UNIQUE INDEX "index_channel_members_on_channel_id_and_user_id" ON "channe
 CREATE TABLE "buffers" (
     "id" INTEGER PRIMARY KEY AUTOINCREMENT,
     "channel_id" INTEGER NOT NULL REFERENCES channels (id) ON DELETE CASCADE,
-    "epoch" INTEGER NOT NULL DEFAULT 0
+    "epoch" INTEGER NOT NULL DEFAULT 0,
+    "latest_operation_epoch" INTEGER,
+    "latest_operation_replica_id" INTEGER,
+    "latest_operation_lamport_timestamp" INTEGER
 );
 
 CREATE INDEX "index_buffers_on_channel_id" ON "buffers" ("channel_id");