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
Up the memory available to daily_hit_totals

* See if we can optimise it down again now it's got the new host_paths JOIN in
Move `daily_hit_totals` to `MATERIALIZED VIEW`

* Before: each hourly refresh > 1min (with 768MB `work_mem`)
* After: each hourly refresh 4s and a lot less code

Conflicts:
	db/schema.rb
	lib/transition/import/daily_hit_totals.rb