Escrowed MPT and gateway_balances

The TokenEscrow amendment will allow tokens to be escrowed.

gateway_balances does not support MPTs. However, when an account is involved in EscrowCreate transaction where MPT is being escrowed, this method throws an internal error. Yet if the account holds some MPT but is not involved in escrow transactions, gateway_balances returns success. It should return success with no data in both cases. This is a bug that occurs due to not excluding the MPT type when totaling the escrowed amount. It's a quick fix in GatewayBalances.cpp, and as an RPC fix, it does not need an amendment.