SKETCH: Renormalise - add host_path_id to hits

* Drop other hits fields that are served by this
* Make the specs pass
* Oh my word import is blocking-slow. Now to optimise.
Optimise hits import UPDATE_FROM_STAGING

* Goes from blocking 3.5mins to about a second for 2000 hits_staging rows in dev
* Still slower than non-normalised on rake benchmark:import:hits at around 20s
* Try INSERTs next
Fix benchmark to acknowledge host_path

* Hits don't point to hosts any more
Check that we really are grouping the host_paths

* We never checked for this before, leading to passing tests but a query
  that could result in dupes