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/pmt_tests.cpp

@@ -48,7 +48,7 @@ BOOST_AUTO_TEST_CASE(pmt_test1)
         }
 
         // calculate actual merkle root and height
-        uint256 merkleRoot1 = block.BuildMerkleTree();
+        uint256 merkleRoot1 = block.ComputeMerkleRoot();
         std::vector<uint256> vTxid(nTx, uint256());
         for (unsigned int j=0; j<nTx; j++)
             vTxid[j] = block.vtx[j].GetHash();