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/node/interfaces.cpp

@@ -377,9 +377,8 @@ public:
     std::unique_ptr<Handler> handleNotifyHeaderTip(NotifyHeaderTipFn fn) override
     {
         return MakeHandler(
-            ::uiInterface.NotifyHeaderTip_connect([fn](SynchronizationState sync_state, const CBlockIndex* block) {
-                fn(sync_state, BlockTip{block->nHeight, block->GetBlockTime(), block->GetBlockHash()},
-                    /* verification progress is unused when a header was received */ 0);
+            ::uiInterface.NotifyHeaderTip_connect([fn](SynchronizationState sync_state, int64_t height, int64_t timestamp, bool presync) {
+                fn(sync_state, BlockTip{(int)height, timestamp, uint256{}}, presync);
             }));
     }
     NodeContext* context() override { return m_context; }