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/clientmodel.cpp

@@ -55,6 +55,7 @@ int ClientModel::getNumConnections(unsigned int flags) const
 
 int ClientModel::getNumBlocks() const
 {
+    LOCK(cs_main);
     return chainActive.Height();
 }
 
@@ -76,6 +77,7 @@ quint64 ClientModel::getTotalBytesSent() const
 
 QDateTime ClientModel::getLastBlockDate() const
 {
+    LOCK(cs_main);
     if (chainActive.Tip())
         return QDateTime::fromTime_t(chainActive.Tip()->GetBlockTime());
     else
@@ -84,6 +86,7 @@ QDateTime ClientModel::getLastBlockDate() const
 
 double ClientModel::getVerificationProgress() const
 {
+    LOCK(cs_main);
     return Checkpoints::GuessVerificationProgress(chainActive.Tip());
 }