wallet: new transaction construction algorithm

It should avoid a lot of the issues sending more than half the
wallet's contents due to change.

Actual output selection is still random. Changing this would
improve the matching of transaction amounts to output sizes,
but may have non obvious effects on blockchain analysis.

Mapped to the new transfer_new command in simplewallet, and
transfer uses the existing algorithm.

To use in RPC, add "new_algorithm: true" in the transfer_split
JSON command. It is not used in the transfer command.

src/wallet/wallet_rpc_server_commands_defs.h

@@ -126,6 +126,7 @@ namespace wallet_rpc
       uint64_t mixin;
       uint64_t unlock_time;
       std::string payment_id;
+      bool new_algorithm;
 
       BEGIN_KV_SERIALIZE_MAP()
         KV_SERIALIZE(destinations)
@@ -133,6 +134,7 @@ namespace wallet_rpc
         KV_SERIALIZE(mixin)
         KV_SERIALIZE(unlock_time)
         KV_SERIALIZE(payment_id)
+        KV_SERIALIZE(new_algorithm)
       END_KV_SERIALIZE_MAP()
     };