api,db: add page attributes and thread locking

db/20180922181651-page-attributes.sql

@@ -0,0 +1,9 @@
+-- Introduces page attributes
+
+CREATE TABLE IF NOT EXISTS pages (
+  domain                   TEXT          NOT NULL                           ,
+  path                     TEXT          NOT NULL                           ,
+  isLocked                 BOOLEAN       NOT NULL  DEFAULT false
+);
+
+CREATE UNIQUE INDEX pagesUniqueIndex ON pages(domain, path);