Make singleton instances on plugins only accessible through getters (#477)

* refactor plugins

* fix fmt

* fix compile err

* fix errors

* expose singletons via sync.once

* re-disable remotelog and webauth plugins

* re-enable autopeering plugin

* Apply suggestions from code review

* Apply suggestions from code review

* add descriptive comments for getters

Co-authored-by: Luca Moser <moser.luca@gmail.com>