Transaction timestamp should have limited lifetime

[vekexasia]

Problem:
Currently if A needs to send a transaction to B and B is in the same network he could:

• MITM
• Collect the signed transaction on its way to a peer

By forcing A to rebroadcast a newly crafted transaction B can later broadcast the collected transaction effectively getting paid twice.

Solution?:
The timestamp field should not be allowed to be too old. Maybe we should do some research and decide a time-window after which a transaction expires and can no longer be accepted by the network.

Another, more complicated, solution might involve adding a new field to each transaction that reflect, somehow, the currenct account status so that one tx excludes the other.