* Do not validate a candidate in candidate selection
The candidate selection subsystem should not validate a candidate, as
this is done by the backing subsystem on a `Second` request. Otherwise
we validate one candidate twice.
* Update candidate-selection.md
* choose the straightforward candidate selection algorithm for now
* add draft implementation of candidate selection
* fix typo in summary
* more properly report misbehaving collators
* describe how CandidateSelection subsystem becomes aware of candidates
* revise candidate selection / collator protocol interaction pattern
* implement rest of candidate selection per the guide
* review: resolve nits
* start writing test suite, harness
* implement first test
* add second test
* implement third test
Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>
Bastian Köcher