add support for index groups (enable sharding)

[postspectacular]

Currently all facts are centrally indexed by the graph's N index vertices (e.g. subject, predicate, object). Not scalable.

Ideas/tasks:

• add index group vertex/entity (set of N index vertices), each providing filter range (or rather predicate fn) of acceptable fact IDs and acts as facade to actual index vertices
• redistribution/re-indexing when adding/removing/failing index groups
• refactor fact vertex adding/removing