GDAX: cancel_after

[DeviaVir]

Superseding #340

Please review:

• @crubb
• @nedievas

[nedievas]

Hi. It would be nice to use similar option for Bitfinex by adding manual cancel to replicate this GDAX order parameter, wouldn't?

[nedievas]

Reading GDAX API docs I found that we miss another option for 'cancel_after' to work as intended:

Time in force
Time in force policies provide guarantees about the lifetime of an order. There are four policies: good till canceled GTC, good till time GTT, immediate or cancel IOC, and fill or kill FOK.
GTT Good till time orders remain open on the book until canceled or the allotted cancel_after is depleted on the matching engine. GTT orders are guaranteed to cancel before any other order is processed after the cancel_after timestamp which is returned by the API. A day is considered 24 hours.

Please confirm that this enhancement is really working before merge.

[DeviaVir]

@nedievas good point, added.

[DeviaVir]

Seems to work. Merging.