Switch CCoinsView and chainstate db from per-txid to per-txout

This patch makes several related changes:
* Changes the CCoinsView virtual methods (GetCoins, HaveCoins, ...)
  to be COutPoint/Coin-based rather than txid/CCoins-based.
* Changes the chainstate db to a new incompatible format that is also
  COutPoint/Coin based.
* Implements reconstruction code for hash_serialized_2.
* Adapts the coins_tests unit tests (thanks to Russell Yanofsky).

A side effect of the new CCoinsView model is that we can no longer
use the (unreliable) test for transaction outputs in the UTXO set
to determine whether we already have a particular transaction.

src/test/test_bitcoin_fuzzy.cpp

@@ -168,8 +168,8 @@ int do_fuzz()
         {
             try
             {
-                CCoins block;
-                ds >> block;
+                Coin coin;
+                ds >> coin;
             } catch (const std::ios_base::failure& e) {return 0;}
             break;
         }