Added multiple active tx21's per address

[marv-engine]

Updated tx21 Update & Cancel messages & behaviors

[dexX7]

I'm with @zathras on this one:

...reference the last 8 bytes of the tx hash, where we already have a unique sell offer identifier? We're talking about adding 16 bytes worth of fields in the proposed so it can't be tx space.

16 bytes to give #n and #y values to help us potentially locate the right offer
8 bytes (last 8 of tx hash) to give us exactly the right offer

[dacoinminster]

See #270 for discussion.

Sorry Marv, I thought we had this settled. I hope we didn't just waste your time . . .

[dexX7]

Hi,

in an attempt to clean up the current specification, I'm going to close old and outstanding pull requests. Please note, it's tagged as "old idea", so the work is not wasted and we can potentially review it at some point later.

Please feel free to resubmit it as new pull request, if you still think it's a good idea.