1to2 format converter needs

[Matthew Scott]

So far:

- Convert all (extent_id, oid) tuples to Placeholder instances in
entity fields and indices.

- For all entities, create a related_entities structure to store sets
of related entities.

- Bump format from 1 to 2.

- Do everything in one schevo.store transaction that either succeeds
or fails.

Discussion topic: I think we should design Schevo 3.1 in a way that
it will -not- open a format 1 database and automatically convert it to
format 2. An option should be explicitly given to
schevo.database.open to do so; a "schevo db upgrade DBFILE" command
line tool would expose this to the command-line.