Fix rebuild-chainstate feature and improve its performance

Previous refactorings broke the ability to rebuild the chainstate by deleting the chainstate
directory, resulting in an incorrect "Incorrect or no genesis block found" error message. Fix
that.

Also, improve the performance of ActivateBestBlockStep by using the skiplist to only discover
a few potential blocks to connect at a time, instead of all blocks forever - as we likely bail
out after connecting a single one anyway.

src/init.cpp

@@ -974,7 +974,7 @@ bool AppInit2(boost::thread_group& threadGroup)
 
                 // If the loaded chain has a wrong genesis, bail out immediately
                 // (we're likely using a testnet datadir, or the other way around).
-                if (!mapBlockIndex.empty() && chainActive.Genesis() == NULL)
+                if (!mapBlockIndex.empty() && mapBlockIndex.count(Params().HashGenesisBlock()) == 0)
                     return InitError(_("Incorrect or no genesis block found. Wrong datadir for network?"));
 
                 // Initialize the block index (no-op if non-empty database was already loaded)