Fix some LoadChainTip-related init-order bugs.

* Move the writing of fTxIndex to LoadBlockIndex - this fixes a
  bug introduced in d6af06d68a where
  InitBlockIndex was writing to fTxIndex which had not yet been
  checked (because LoadChainTip hadn't yet initialized the
  chainActive, which would otherwise have resulted in
  InitBlockIndex being a NOP), allowing you to modify -txindex
  without reindex, potentially corrupting your chainstate!

* Rename InitBlockIndex to LoadGenesisBlock, which is now a more
  natural name for it. Also check mapBlockIndex instead of
  chainActive, fixing a bug where we'd write the genesis block out
  on every start.

src/test/test_bitcoin.cpp

@@ -72,7 +72,7 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
         pblocktree = new CBlockTreeDB(1 << 20, true);
         pcoinsdbview = new CCoinsViewDB(1 << 23, true);
         pcoinsTip = new CCoinsViewCache(pcoinsdbview);
-        if (!InitBlockIndex(chainparams)) {
+        if (!LoadGenesisBlock(chainparams)) {
             throw std::runtime_error("InitBlockIndex failed.");
         }
         {