Solve chainActive-related locking issues

- In wallet and GUI code LOCK cs_main as well as cs_wallet when
  necessary
- In main.cpp SendMessages move the TRY_LOCK(cs_main) up, to encompass the call
  to IsInitialBlockDownload.
- Make ActivateBestChain, AddToBlockIndex, IsInitialBlockDownload,
  InitBlockIndex acquire the cs_main lock

Fixes #3997

src/qt/transactiontablemodel.h

@@ -69,7 +69,6 @@ private:
     WalletModel *walletModel;
     QStringList columns;
     TransactionTablePriv *priv;
-    int cachedNumBlocks;
 
     QString lookupAddress(const std::string &address, bool tooltip) const;
     QVariant addressColor(const TransactionRecord *wtx) const;