* Add the deriver to the ValidPaths table. In principle we could now

store all the derivers of a path efficiently.  But that opens a big
  can of worms with respect to garbage collector semantics.

src/libstore/schema.sql

@@ -4,7 +4,8 @@ create table if not exists ValidPaths (
     id               integer primary key autoincrement not null,
     path             text unique not null,
     hash             text not null,
-    registrationTime integer not null
+    registrationTime integer not null,
+    deriver          text
 );
 
 create table if not exists Refs (