Read DEFAULT_LOCKUP_PERIOD_DAYS from contract instead of environment variable

[pyropy]

IIRC, we agreed with @pyropy to use the constant so that we can ship this sooner. DEFAULT_LOCKUP_PERIOD_DAYS is unlikely to change often.

How about creating a follow-up tech-debt issue for addressing this later? If we feel strongly about this, we can add the new issue to the GA launch backlog.

Originally posted by @bajtos in #324

[DarkLord017]

can i work on this @pyropy

[pyropy]

@DarkLord017 Let's first close the open tickets before picking up new ones.