Revert "Revert "optimizations meant to streamline location import""

This reverts commit 3d676d59f15f140113a937bca17757559797640d.
responses to pull request comments and bugfix to add function
hopefully final bugfix for LocationCache
add __repr__ to locations
remove global uniqueness constraint on site_code
use location.path
Merge pull request #616 from dimagi/location-upload-optimize

Location upload optimize