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/sqlite.hh

@@ -99,8 +99,8 @@ void handleSQLiteBusy(const SQLiteBusy & e);
 
 /* Convenience function for retrying a SQLite transaction when the
    database is busy. */
-template<typename T>
-T retrySQLite(std::function<T()> fun)
+template<typename T, typename F>
+T retrySQLite(F && fun)
 {
     while (true) {
         try {