store: design model for writing to other stores

[adg]

We need to design the mechanism that enables writing to stores other than your primary store.

For example, if a file system is linked into your root, you should be able to write into that file system and have the references end up in that file system's store (not whatever store you have specified in your rc).

This also means we need to design directory linking.

[robpike]

Long since done.