User `import_bulk()` instead of `insert_many()`

[waxlamp]

import_bulk() is faster than insert_many() because it doesn't track error conditions as finely, simply raising an exception on the first insertion failure, rather than doing all the inserts and individually tracking which insertions failed. For us, we would prefer to fail early in this case, since we wish to do these imports as all-or-nothing in general.

See original discussion.