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/main.cpp

@@ -2595,7 +2595,7 @@ bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW, bo
     // Check the merkle root.
     if (fCheckMerkleRoot) {
         bool mutated;
-        uint256 hashMerkleRoot2 = block.BuildMerkleTree(&mutated);
+        uint256 hashMerkleRoot2 = block.ComputeMerkleRoot(&mutated);
         if (block.hashMerkleRoot != hashMerkleRoot2)
             return state.DoS(100, error("CheckBlock(): hashMerkleRoot mismatch"),
                              REJECT_INVALID, "bad-txnmrklroot", true);