Add "delivered_amount" synthetic field to Escrow and Payment Channel

[carlhua]

For the last couple of years the account_tx RPC command has returned a synthetic field: delivered_amount. That field provides a standard way for gateways to know how much someone's balance changed.

Since then, we've added two new kinds of transactions that can transfer funds: escrow and payment channels. We should delivered_amount to these transaction types and any others that can deliver value but don't currently show delivered_amount in the metadata.

~~While we're in there, this may be the right time to remove the deprecated implementation of account_tx. (See file AccountTxOld.cpp.) There are two implementations. The deprecated implementation is now 2 years old. We need to determine whether the old implementation is safe to remove and, if it is safe, remove it.~~

Exported from RIPD-1573

[mDuo13]

The delivered_amount bits are related to #3468 and #3372.

The deprecated account_tx switch is issue #2926 (fixed).

[mDuo13]

Was reminded today that this still isn't the case. It's inconvenient that delivered_amount isn't available on EscrowFinish transactions.