[validation] Add CValidationState subclasses

Split CValidationState into TxValidationState and BlockValidationState
to store validation results for transactions and blocks respectively.

src/init.cpp

@@ -712,7 +712,7 @@ static void ThreadImport(std::vector<fs::path> vImportFiles)
     }
 
     // scan for better chains in the block chain database, that are not yet connected in the active best chain
-    CValidationState state;
+    BlockValidationState state;
     if (!ActivateBestChain(state, chainparams)) {
         LogPrintf("Failed to connect best block (%s)\n", FormatStateMessage(state));
         StartShutdown();