Fixed #6634: Asset Import History fixes and optimizations

[herroworrd]

I took a quick stab at fixing asset history imports.

Assumptions made:
-Asset history is coming from a previous management system.
-It isn't historical unless it has both a checkout and a checkin (sysadmin can fudge a checkin date if they don't have this information in previous system)
-The history importer no longer assigns assets (this can be done using the regular importer)

Optimizations made:
-Removed second iteration of asset import list, including all business attempting to 'magically' create past checkin dates
-Removed creation of, potentially giant, $items array
-Removed username matching schemes and only use that which is configured in settings
-CSV is sorted using league/csv statement filters (in post) on username to reduce db or cache calls
-Asset and User query results (both hits and misses) are cached (for 1 minute) to avoid duplicate queries
-Some better controls around date conversions included

[snipe]

Thanks!