Support network access/partitioning

[dw]

This has been discussed a little via e-mail, but it's too far in the future to think about, so marking unlikely. Mostly covers the following old TODOs:

1. Support "read-only" :py:class:Index object
2. Minimalist validating+indexing network server module
3. Engine or :py:class:Collection that implements caching on top of another
4. Engine that distributes keyspace using configurable scheme

See also issue #7