Use async commits

* Speeds up smaller transactions
* Returns before write-ahead log is persisted to disk
* Gives up the D (durability) in ACID - could result in data loss if the
  server crashed, but in the case of hits this would almost
  immediately be re-imported and in the case of bulk mapping
  adds would be repeatable.
Remove unnecessary MINs
Don't rely on a function loading hits

* Computed functions can't work against an index
Don't update hits_mappings_relations

* (that haven't changed)