add support for 'echo' on Flow frames/events

Flow frames have an 'echo' field [1], used to request peers send their updated state. This would be useful to e.g attempt stopping a link without 'drain', or requesting a Flow response is sent when also 'drain'ing (since Proton does not send one if all the credit has already been used when the drain is actioned).

Currently this field is not inspected by proton itself, or exposed in any way that an application could inspect this (or it must be said, set it) itself, so it isnt currently possible to honour an echo request from a [non-Proton] peer.

[1] http://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-transport-v1.0-os.html#type-flow