refactoring: FlushStateToDisk -> CChainState

Also renames global methods for clarity:

- ::FlushStateToDisk() -> CChainState::ForceFlushStateToDisk()
  - This performs an unconditional flush.

- ::PruneAndFlush() -> CChainState::PruneAndFlush()

src/init.cpp

@@ -259,7 +259,7 @@ void Shutdown(InitInterfaces& interfaces)
 
     // FlushStateToDisk generates a ChainStateFlushed callback, which we should avoid missing
     if (pcoinsTip != nullptr) {
-        FlushStateToDisk();
+        ::ChainstateActive().ForceFlushStateToDisk();
     }
 
     // After there are no more peers/RPC left to give us new data which may generate
@@ -275,7 +275,7 @@ void Shutdown(InitInterfaces& interfaces)
     {
         LOCK(cs_main);
         if (pcoinsTip != nullptr) {
-            FlushStateToDisk();
+            ::ChainstateActive().ForceFlushStateToDisk();
         }
         pcoinsTip.reset();
         pcoinscatcher.reset();
@@ -1692,7 +1692,7 @@ bool AppInitMain(InitInterfaces& interfaces)
         nLocalServices = ServiceFlags(nLocalServices & ~NODE_NETWORK);
         if (!fReindex) {
             uiInterface.InitMessage(_("Pruning blockstore..."));
-            PruneAndFlush();
+            ::ChainstateActive().PruneAndFlush();
         }
     }