Clean up local mode engine objects using snapshots

Snapshot is collected to compare with engine state when local mode is turned off.

Change-type: minor
Signed-off-by: Roman Mazur <roman@balena.io>

src/migrations/20190619152500-engine-snapshot.js

@@ -0,0 +1,11 @@
+
+exports.up = (knex, Promise) => {
+	return knex.schema.createTable('engineSnapshot', (t) => {
+		t.string('snapshot'); // Engine snapshot encoded as JSON.
+		t.string('timestamp'); // When the snapshot was created.
+	});
+}
+
+exports.down = (knex, Promise) => {
+	return Promise.reject(new Error('Not Implemented'));
+}