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/net_processing.cpp

@@ -911,12 +911,11 @@ bool static AlreadyHave(const CInv& inv) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
                 recentRejects->reset();
             }
 
-            // Use pcoinsTip->HaveCoinsInCache as a quick approximation to exclude
-            // requesting or processing some txs which have already been included in a block
             return recentRejects->contains(inv.hash) ||
                    mempool.exists(inv.hash) ||
                    mapOrphanTransactions.count(inv.hash) ||
-                   pcoinsTip->HaveCoinsInCache(inv.hash);
+                   pcoinsTip->HaveCoinsInCache(COutPoint(inv.hash, 0)) || // Best effort: only try output 0 and 1
+                   pcoinsTip->HaveCoinsInCache(COutPoint(inv.hash, 1));
         }
     case MSG_BLOCK:
     case MSG_WITNESS_BLOCK: