feat: supporting unlocked with withdrawal period

[wei3erHase]

The underlying codebase requires the user to decide at lockup the Date of withdrawal.
The intended behaviour, is that the user can decide to lock the tokens in an "unlocked with withdrawal period" mode, in which it would require the user to first "initiate the withdrawal", to be able to actually withdraw the tokens (e.g.) 7 days later.

This is achieved by:

• reverting on withdrawal when locking Date is 0
• adding a new method, identical in arguments to "unlock", that would revert if Date != 0
• when user calls this method, the locking Date should be now() + 7 days