move-mostly: Make pindexBestHeader a ChainMan member

[META] In the next commit, we move the clearing of pindexBestHeader to
       ChainstateManager::Unload()

src/node/interfaces.cpp

@@ -212,9 +212,10 @@ public:
     bool getHeaderTip(int& height, int64_t& block_time) override
     {
         LOCK(::cs_main);
-        if (::pindexBestHeader) {
-            height = ::pindexBestHeader->nHeight;
-            block_time = ::pindexBestHeader->GetBlockTime();
+        auto best_header = chainman().pindexBestHeader;
+        if (best_header) {
+            height = best_header->nHeight;
+            block_time = best_header->GetBlockTime();
             return true;
         }
         return false;