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.hh

@@ -255,6 +255,10 @@ public:
 
     const Setting<bool> isTrusted{this, false, "trusted", "whether paths from this store can be used as substitutes even when they lack trusted signatures"};
 
+    Setting<int> priority{this, 0, "priority", "priority of this substituter (lower value means higher priority)"};
+
+    Setting<bool> wantMassQuery{this, false, "want-mass-query", "whether this substituter can be queried efficiently for path validity"};
+
 protected:
 
     struct State
@@ -422,8 +426,6 @@ public:
     virtual void querySubstitutablePathInfos(const StorePathSet & paths,
         SubstitutablePathInfos & infos) { return; };
 
-    virtual bool wantMassQuery() { return false; }
-
     /* Import a path into the store. */
     virtual void addToStore(const ValidPathInfo & info, Source & narSource,
         RepairFlag repair = NoRepair, CheckSigsFlag checkSigs = CheckSigs,
@@ -648,11 +650,6 @@ public:
         return 0;
     };
 
-    /* Get the priority of the store, used to order substituters. In
-       particular, binary caches can specify a priority field in their
-       "nix-cache-info" file. Lower value means higher priority. */
-    virtual int getPriority() { return 0; }
-
     virtual Path toRealPath(const Path & storePath)
     {
         return storePath;