Instead of first collecting all keys and then batch deleting them,
we will do the incremental delete _online_ per max allowed batch.
Doing this prevents frequent allocations for large S3 keyspaces
and OOM-kills that might happen as a result of those.
This commit introduces storagedriver.Errors type that allows to return
multierrors as a single error from any storage driver implementation.
Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>