Add AssertLockHeld for cs_main to ChainActive-using functions

All functions that use ChainActive but do not aquire the cs_main
lock themselves, need to be called with the cs_main lock held.

This commit adds assertions to all externally callable functions
that use chainActive or chainMostWork.

This will flag usages when built with -DDEBUG_LOCKORDER.

src/qt/transactiondesc.cpp

@@ -20,6 +20,7 @@
 
 QString TransactionDesc::FormatTxStatus(const CWalletTx& wtx)
 {
+    AssertLockHeld(cs_main);
     if (!IsFinalTx(wtx, chainActive.Height() + 1))
     {
         if (wtx.nLockTime < LOCKTIME_THRESHOLD)