sqldb: add table to track custom SQL migrations

This commit adds the migration_tracker table which we'll use to track if
a custom migration has already been done.

sqldb/sqlc/queries/migration.sql

@@ -0,0 +1,21 @@
+-- name: SetMigration :exec
+INSERT INTO
+  migration_tracker (version, migration_time) 
+VALUES ($1, $2);
+
+-- name: GetMigration :one
+SELECT
+  migration_time
+FROM
+  migration_tracker
+WHERE
+  version = $1;
+
+-- name: GetDatabaseVersion :one
+SELECT
+  version
+FROM
+  migration_tracker
+ORDER BY
+  version DESC
+LIMIT 1;