Audit-fixes

[auryn-macmillan]

This PR implements fixes for some minor issues identified in an audit.

• removes jump at cliffTime
• eliminates griefiing attack vector on changeAddress
• implements tests for the proposed changes

[jfschwarz]

Cliff vesting plans, as commonly offered for stock options, usually have that "jump" when hitting the cliff. So I think the original implementation is more intuitive.

[auryn-macmillan]

Cliff vesting plans, as commonly offered for stock options, usually have that "jump" when hitting the cliff. So I think the original implementation is more intuitive.

Yeah, after thinking about this more I actually think the original implementation is correct. initialAmount is just a somewhat novel feature that results in the potential for what is essentially two cliffs.