Avoid N+1 query for channels with notes changes

Also, start work on new timing for recording observed notes edits.

Co-authored-by: Mikayla <mikayla@zed.dev>

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

@@ -296,6 +296,7 @@ CREATE TABLE "observed_buffer_edits" (
     "buffer_id" INTEGER NOT NULL REFERENCES buffers (id) ON DELETE CASCADE,
     "epoch" INTEGER NOT NULL,
     "lamport_timestamp" INTEGER NOT NULL,
+    "replica_id" INTEGER NOT NULL,
     PRIMARY KEY (user_id, buffer_id)
 );