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/primitives/block.h

@@ -77,9 +77,6 @@ public:
     // network and disk
     std::vector<CTransaction> vtx;
 
-    // memory only
-    mutable std::vector<uint256> vMerkleTree;
-
     CBlock()
     {
         SetNull();
@@ -103,7 +100,6 @@ public:
     {
         CBlockHeader::SetNull();
         vtx.clear();
-        vMerkleTree.clear();
     }
 
     CBlockHeader GetBlockHeader() const
@@ -118,14 +114,12 @@ public:
         return block;
     }
 
-    // Build the in-memory merkle tree for this block and return the merkle root.
+    // Build the merkle tree for this block and return the merkle root.
     // If non-NULL, *mutated is set to whether mutation was detected in the merkle
     // tree (a duplication of transactions in the block leading to an identical
     // merkle root).
-    uint256 BuildMerkleTree(bool* mutated = NULL) const;
+    uint256 ComputeMerkleRoot(bool* mutated = NULL) const;
 
-    std::vector<uint256> GetMerkleBranch(int nIndex) const;
-    static uint256 CheckMerkleBranch(uint256 hash, const std::vector<uint256>& vMerkleBranch, int nIndex);
     std::string ToString() const;
 };