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/qt/transactiondesc.cpp

@@ -293,13 +293,12 @@ QString TransactionDesc::toHTML(CWallet *wallet, CWalletTx &wtx, TransactionReco
         {
             COutPoint prevout = txin.prevout;
 
-            CCoins prev;
-            if(pcoinsTip->GetCoins(prevout.hash, prev))
+            Coin prev;
+            if(pcoinsTip->GetCoins(prevout, prev))
             {
-                if (prevout.n < prev.vout.size())
                 {
                     strHTML += "<li>";
-                    const CTxOut &vout = prev.vout[prevout.n];
+                    const CTxOut &vout = prev.out;
                     CTxDestination address;
                     if (ExtractDestination(vout.scriptPubKey, address))
                     {