sqlite re-uses ID for columns not marked as autoincrement, so use that.

fixes #202

pkg/datastore/sqlstore/migrations/sqlite3/26_sites_id_autoinc.sql

@@ -0,0 +1,19 @@
+-- +migrate Up
+DROP TABLE IF EXISTS sites_old;
+ALTER TABLE sites RENAME TO sites_old;
+CREATE TABLE sites (
+    `id` INTEGER PRIMARY KEY AUTOINCREMENT,
+    `tracking_id` VARCHAR(8) UNIQUE,
+    `name` VARCHAR(100) NOT NULL
+);
+INSERT INTO sites SELECT `id`, `tracking_id`, `name` FROM sites_old;
+
+-- +migrate Down
+DROP TABLE IF EXISTS sites_old;
+ALTER TABLE sites RENAME TO sites_old;
+CREATE TABLE sites (
+    `id` INTEGER PRIMARY KEY,
+    `tracking_id` VARCHAR(8) UNIQUE,
+    `name` VARCHAR(100) NOT NULL
+);
+INSERT INTO sites SELECT `id`, `tracking_id`, `name` FROM sites_old;