Use advisory lock when assigning sequence IDs (#401)

Hoping for this to be an invisible change - relying on existing unit tests for verification.

Whenever we add a table that relies on a sequence for ordering, we should consider adding a Postgres stored function that performs insertions by taking an advisory lock first. This ensures that inserts on the table are [processed serially](https://espadrine.github.io/blog/posts/two-postgresql-sequence-misconceptions.html#Order_violation).

For these stored functions, I've matched the params and param ordering to the existing queries. I also allowed unique constraint errors to bubble up rather than specifying `ON CONFLICT`, to match the existing queries.

The `dev/up` script also generates the protos, and I don't see a convenient way to exclude the protos for the replication project. However, I assume that at some point we will need them on this server, for forwards-compatibility purposes.

pkg/migrations/mls/20240829001344_serial-ids.down.sql

@@ -0,0 +1,6 @@
+DROP FUNCTION insert_group_message;
+
+DROP FUNCTION insert_welcome_message;
+
+DROP FUNCTION insert_inbox_log;
+

pkg/migrations/mls/20240829001344_serial-ids.up.sql

@@ -0,0 +1,44 @@
+CREATE FUNCTION insert_group_message(group_id BYTEA, data BYTEA, group_id_data_hash BYTEA)
+	RETURNS SETOF group_messages
+	AS $$
+BEGIN
+	-- Ensures that the generated sequence ID matches the insertion order
+	-- Only released at the end of the enclosing transaction - beware if called within a long transaction
+	PERFORM
+		pg_advisory_xact_lock(hashtext('group_messages_sequence'), hashtext(encode(group_id, 'hex')));
+	RETURN QUERY INSERT INTO group_messages(group_id, data, group_id_data_hash)
+		VALUES(group_id, data, group_id_data_hash)
+	RETURNING
+		*;
+END;
+$$
+LANGUAGE plpgsql;
+
+CREATE FUNCTION insert_welcome_message(installation_key BYTEA, data BYTEA, installation_key_data_hash BYTEA, hpke_public_key BYTEA)
+	RETURNS SETOF welcome_messages
+	AS $$
+BEGIN
+	PERFORM
+		pg_advisory_xact_lock(hashtext('welcome_messages_sequence'), hashtext(encode(installation_key, 'hex')));
+	RETURN QUERY INSERT INTO welcome_messages(installation_key, data, installation_key_data_hash, hpke_public_key)
+		VALUES(installation_key, data, installation_key_data_hash, hpke_public_key)
+	RETURNING
+		*;
+END;
+$$
+LANGUAGE plpgsql;
+
+CREATE FUNCTION insert_inbox_log(inbox_id BYTEA, server_timestamp_ns BIGINT, identity_update_proto BYTEA)
+	RETURNS SETOF inbox_log
+	AS $$
+BEGIN
+	PERFORM
+		pg_advisory_xact_lock(hashtext('inbox_log_sequence'), hashtext(encode(inbox_id, 'hex')));
+	RETURN QUERY INSERT INTO inbox_log(inbox_id, server_timestamp_ns, identity_update_proto)
+		VALUES(inbox_id, server_timestamp_ns, identity_update_proto)
+	RETURNING
+		*;
+END;
+$$
+LANGUAGE plpgsql;
+

pkg/mls/store/queries.sql

@@ -59,10 +59,10 @@ RETURNING
 	*;
 
 -- name: InsertInboxLog :one
-INSERT INTO inbox_log(inbox_id, server_timestamp_ns, identity_update_proto)
-	VALUES (decode(@inbox_id, 'hex'), @server_timestamp_ns, @identity_update_proto)
-RETURNING
-	sequence_id;
+SELECT
+	sequence_id
+FROM
+	insert_inbox_log(decode(@inbox_id, 'hex'), @server_timestamp_ns, @identity_update_proto);
 
 -- name: RevokeAddressFromLog :exec
 UPDATE
@@ -111,16 +111,16 @@ WHERE
 	id = ANY (@installation_ids::BYTEA[]);
 
 -- name: InsertGroupMessage :one
-INSERT INTO group_messages(group_id, data, group_id_data_hash)
-	VALUES ($1, $2, $3)
-RETURNING
-	*;
+SELECT
+	*
+FROM
+	insert_group_message(@group_id, @data, @group_id_data_hash);
 
 -- name: InsertWelcomeMessage :one
-INSERT INTO welcome_messages(installation_key, data, installation_key_data_hash, hpke_public_key)
-	VALUES ($1, $2, $3, $4)
-RETURNING
-	*;
+SELECT
+	*
+FROM
+	insert_welcome_message(@installation_key, @data, @installation_key_data_hash, @hpke_public_key);
 
 -- name: GetAllGroupMessages :many
 SELECT