for queries, assume a fresh instance if 'since' is not passed

Summary:
This is the only sensible thing to do.

Fix checks for `time_generator` and friends, and drop checks for `query->since_spec` practically everywhere since it no longer makes sense.

Depends on D915984.

Depends on D915989.

Test Plan: `arc unit --everything` with the added test.

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D913916

Task ID: 2689998

--HG--
extra : rebase_source : c72e8582216150715b1f093eca8744751e906b8d
extra : amend_source : 8e6d782321682220c8392985ce5f88d50954b776