mastopurge never pulled more than one page of toots

[mariow]

Prior to this change prevmaxid was always updated after updating maxid and thus both were always identical. This in turn caused the loop to break after the first run (line 362 of mastopurge.go).

Updating prevmaxid before maxid fixed the issue for me and mastopurge now works fine deleting old toots that are not on the first results page.