Distance Oracle (!105)

* Fix distance-oracle EvaluationPool type

* Remove instanciation and dep to certification

* Doc comments, make max size params const

* Fix default distance dir

* Rename pool accessors

* doc add READMEs

* fix(distance): Remember account_id who reserved

* Log, comments, crate author

* feat: distance

* integration tests, expiration

* fixes & working end2end tests

* fix(distance): max_depth, compute min_certs_for_referee

* fix(distance): add distance pallet to gtest, g1

* test(distance): WiP end2end test

* feat: distance