Add "nix verify-paths" command

Unlike "nix-store --verify-path", this command verifies signatures in
addition to store path contents, is multi-threaded (especially useful
when verifying binary caches), and has a progress indicator.

Example use:

$ nix verify-paths --store https://cache.nixos.org -r $(type -p thunderbird)
...
[17/132 checked] checking ‘/nix/store/rawakphadqrqxr6zri2rmnxh03gqkrl3-autogen-5.18.6’

src/nix/progress-bar.hh

@@ -0,0 +1,49 @@
+#pragma once
+
+#include "sync.hh"
+#include "util.hh"
+
+namespace nix {
+
+class ProgressBar
+{
+private:
+    struct State
+    {
+        std::string status;
+        bool done = false;
+        std::list<std::string> activities;
+    };
+
+    Sync<State> state;
+
+public:
+
+    ProgressBar();
+
+    ~ProgressBar();
+
+    void updateStatus(const std::string & s);
+
+    void done();
+
+    class Activity
+    {
+        friend class ProgressBar;
+    private:
+        ProgressBar & pb;
+        std::list<std::string>::iterator it;
+        Activity(ProgressBar & pb, const FormatOrString & fs);
+    public:
+        ~Activity();
+    };
+
+    Activity startActivity(const FormatOrString & fs);
+
+private:
+
+    void render(State & state_);
+
+};
+
+}