Use BlockStatus's modifier function to update status's value.

Summary:
As per title. Most of the code doesn't need to know about the internal layour of the status's field.

Depends on D1461

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1462