Do not store Merkle branches in the wallet.

Assume that when a wallet transaction has a valid block hash and transaction position
in it, the transaction is actually there. We're already trusting wallet data in a
much more fundamental way anyway.

To prevent backward compatibility issues, a new record is used for storing the
block locator in the wallet. Old wallets will see a wallet file synchronized up
to the genesis block, and rescan automatically.

src/test/miner_tests.cpp

@@ -87,7 +87,7 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
         pblock->vtx[0] = CTransaction(txCoinbase);
         if (txFirst.size() < 2)
             txFirst.push_back(new CTransaction(pblock->vtx[0]));
-        pblock->hashMerkleRoot = pblock->BuildMerkleTree();
+        pblock->hashMerkleRoot = pblock->ComputeMerkleRoot();
         pblock->nNonce = blockinfo[i].nonce;
         CValidationState state;
         BOOST_CHECK(ProcessNewBlock(state, NULL, pblock, true, NULL));