Make FindBlockByHeight constant-time.

Remove the pnext pointer in CBlockIndex, and replace it with a
vBlockIndexByHeight vector (no effect on memory usage). pnext can
now be replaced by vBlockIndexByHeight[nHeight+1], but
FindBlockByHeight becomes constant-time.

This also means the entire mapBlockIndex structure and the block
index entries in it become purely blocktree-related data, and
independent from the currently active chain, potentially allowing
them to be protected by separate mutexes in the future.

src/wallet.cpp

@@ -780,7 +780,7 @@ int CWallet::ScanForWalletTransactions(CBlockIndex* pindexStart, bool fUpdate)
                 if (AddToWalletIfInvolvingMe(tx.GetHash(), tx, &block, fUpdate))
                     ret++;
             }
-            pindex = pindex->pnext;
+            pindex = pindex->GetNextInMainChain();
         }
     }
     return ret;