[ENSCORESW-840]. seq_region_start from db in case of circular chromosome
[ENSCORESW-840]. seq region boundaries are fetched from the db (for slices stored in the db) in case
of circular chromosomes. This is because it is impossible to recompute the seq_region_start|end for
a feature in a circular slice since the transformation operated in method _objs_from_sth in the
corresponding adaptor is not invertible.
For some of the features (e.g. those in the Bio::EnsEMBL::FeaturePair hierarchy), we can't query
the db and the methods seq_region_start|end return undef even if the feature is on a slice.