Change the import queue traits to take &mut self instead of &self (#3058)

* SharedFinalityProofRequestBuilder -> BoxFinalityProofRequestBuilder

* SharedThings -> BoxThings

* Fix tests

* build_request_data now takes &mut self

* The other traits now also take &mut self

* More or less fix tests

* Fix tests

* Fix more tests

* Moar tests

* Don't call make_block_import multiple time

* Fix doctest