Allow parameters in store URIs

This is to allow store-specific configuration,
e.g. s3://my-cache?compression=bzip2&secret-key=/path/to/key.

src/libstore/local-store.cc

@@ -229,6 +229,12 @@ LocalStore::~LocalStore()
 }
 
 
+std::string LocalStore::getUri()
+{
+    return "local";
+}
+
+
 int LocalStore::getSchema()
 {
     int curSchema = 0;