Add priority setting to stores

This allows overriding the priority of substituters, e.g.

  $ nix-store --store ~/my-nix/ -r /nix/store/df3m4da96d84ljzxx4mygfshm1p0r2n3-geeqie-1.4 \
    --substituters 'http://cache.nixos.org?priority=100 daemon?priority=10'

Fixes #3264.

src/libstore/store-api.cc

@@ -975,7 +975,7 @@ std::list<ref<Store>> getDefaultSubstituters()
             addStore(uri);
 
         stores.sort([](ref<Store> & a, ref<Store> & b) {
-            return a->getPriority() < b->getPriority();
+            return a->priority < b->priority;
         });
 
         return stores;