Revisit MySQL `bulk_insert` implementation

[m4tx]

In #414 we've added a support for bulk inserting multiple records into DB at once.

The MySQL implementation is pretty fragile, as it depends on the fact that the newly added records usually have consecutive IDs, but this doesn't necessarily have to be the case. It would be good to have a look whether it can be improved somehow.

(It might be worth considering dropping support for MySQL and focusing on MariaDB instead; given it supports RETURNING clause, it might fix more than one issue.)