Add a consistency check for the block chain data structures

This adds a -checkblockindex (defaulting to true for regtest), which occasionally
does a full consistency check for mapBlockIndex, setBlockIndexCandidates, chainActive, and
mapBlocksUnlinked.

src/test/test_bitcoin.cpp

@@ -29,6 +29,7 @@ extern void noui_connect();
 BasicTestingSetup::BasicTestingSetup()
 {
         fPrintToDebugLog = false; // don't want to write to debug.log file
+        fCheckBlockIndex = true;
         SelectParams(CBaseChainParams::MAIN);
 }
 BasicTestingSetup::~BasicTestingSetup()