add timelock checks in lockToken

As of now, this doesn't pose any risk but needs to be resolved before making the protocol open access.

TMS-3
File(s):
sip10swap_v3.clar
Recommendation: To ensure the timelock makes sense, add a validation check in the lockToken function, that possibly includes a minimum lock time:
(asserts! (> timelock (+ block-height minimum-timelock) (err err-invalid-timelock))
