Index blocks by number in the DB (#890)

* resolve #780

* in import tx insert after retracting so blocks don't disappear

* adapt db/cache to changes to db/utils

* db/light: remove old hash lookup insertion

* renames

* improve naming

* improve naming

* db/utils: improve docs for block lookup key stuff

* db/light/note_finalized: adapt to changes for issue #780

* db/light/cht/tests: add assertions for HASH_LOOKUP

* simplify

addresses
https://github.com/paritytech/substrate/pull/890#discussion_r226009746

* break long line

addresses
https://github.com/paritytech/substrate/pull/890#discussion_r226002211

* Whitespace