expiry should be a time.Duration

Based on discussion i.e. lockTime is a duration, and I think expiry throughout the SDK should also use it.

This affects APIs like

func (c *Collection) GetAndTouch(id string, expiry uint32, opts *GetAndTouchOptions) (docOut *GetResult, errOut error)

but also replace options, GetResult etc .. so if it is changed the whole code needs to be audited. (not filing another ticket for simplicity)