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/main.h

@@ -123,6 +123,7 @@ extern bool fReindex;
 extern int nScriptCheckThreads;
 extern bool fTxIndex;
 extern bool fIsBareMultisigStd;
+extern bool fCheckBlockIndex;
 extern unsigned int nCoinCacheSize;
 extern CFeeRate minRelayTxFee;