Bug 31740: Remove some unnecessary RemoteSettings instances
More concretely, SearchService.jsm 'hijack-blocklists' and url-classifier-skip-urls. Avoid creating instance for 'anti-tracking-url-decoration'. If prefs are disabling their usage, avoid creating instances for 'cert-revocations' and 'intermediates'. Do not ship JSON dumps for collections we do not expect to need. For the ones in the 'main' bucket, this prevents them from being synced unnecessarily (the code in remote-settings does so for collections in the main bucket for which a dump or local data exists). For the collections in the other buckets, we just save some size by not shipping their dumps. We also clear the collections database on the v2 -> v3 migration.