Refactor to use one type of time - time.time() - and also using UNIX timestamp everywhere.

`creation_date` in the mentorship relation (app/api/dao/mentorship_relation.py line) which is using .timestamp() after datetime.utcnow() where as in the `email verification date` for user register (app/api/dao/user.py), only uses datetime.utcnow() without .timestamp().

**To Reproduce**

Steps to reproduce the behavior:

[This PR comment](https://github.com/anitab-org/mentorship-backend/pull/556#issuecomment-631427249) has a clear description of the issue

**Expected behavior**
All datetimes used (across all endpoints should be standardized).