Enable fast withdrawals from plasma chain.

[paulperegud]

Reason - without fast withdrawals plasma chain is not attractive for payments. It would work without friction only if everyone would be inside.

The design doc by Kelvin:

https://github.com/omisego/research/blob/master/plasma/simple_fast_withdrawals.md

[boolafish]

POC under going: https://github.com/omgnetwork/plasma-contracts/tree/master/plasma_framework/contracts/poc/fast_exits