Because of performance concerns and infrastructure limitations, the
checks which verify that all new LFS pointers have matching LFS objects
is only ever checking the first pushed reference. This is easy to
circumvent by just introducing the LFS pointer with the second
reference (this isn't quite true anymore given that LFS pointer checks
use the object quarantine directory to enumerate LFS pointers nowadays,
which always goes over all pushed objects and ignores refs completely).

Now that we have the infrastructure to easily perform checks over
batched changes, this commit converts the LfsCheck class to be
implemented on top of a new BaseBulkChecker. With this change, we now
have all reference updates available at once and can thus efficiently
query the repository for any new LFS pointers introduced by any one of
the references. This makes for a cleaner design given that we don't need
the `skip_lfs_integrity_check` variable anymore, and is also safer to
use.

Changelog: fixed