make multibackend caps work as advertised

The multibackend performs set intersection to determine intersecting capabilities
of all sub-backends but it used the empty set as `Acc0`, which resulted in the empty set
always. This addresses this issue by using ordsets:intersection/1 instead.

~~Note: this has the side-effect of making the multi-backend index capable if all
sub-backends support indexing (e.g. some mix of memory and leveldb).~~ (the multibackend supported indexing due to the fact that `capabilities/2` returned the proper values based on the sub-backend's capabilities, this just fixes an inconsistency).

Addresses #507