Make validation interface capable of signalling header presync

This makes a number of changes:
- Get rid of the verification_progress argument in the node interface
  NotifyHeaderTip (it was always 0.0).
- Instead of passing a CBlockIndex* in the UI interface's NotifyHeaderTip,
  send separate height, timestamp fields. This is becuase in headers presync,
  no actual CBlockIndex object is available.
- Add a bool presync argument to both of the above, to identify signals
  pertaining to the first headers sync phase.

src/interfaces/node.h

@@ -260,7 +260,7 @@ public:
 
     //! Register handler for header tip messages.
     using NotifyHeaderTipFn =
-        std::function<void(SynchronizationState, interfaces::BlockTip tip, double verification_progress)>;
+        std::function<void(SynchronizationState, interfaces::BlockTip tip, bool presync)>;
     virtual std::unique_ptr<Handler> handleNotifyHeaderTip(NotifyHeaderTipFn fn) = 0;
 
     //! Get and set internal node context. Useful for testing, but not