v0.6.0

Added (breaking)

OutboxStore::rekey() renames the entity every queued mutation refers to.

A create carries a handle the device made up for itself, and the server answers with the name it gave the record. Everything queued behind that create still refers to the handle — left alone, each of those is a write to a record that does not exist.

Mutation identities are untouched: renaming what a write targets is not a new write, and giving it a new id would let the server apply it twice. Covered by a parity test that runs against both shipped stores.

Only the key is renamed. A field value holding the handle — a child carrying its parent's id — belongs to the application, and no store can know which of its fields are references, so Outbox::rekey() leaves that to the caller rather than doing it half way.

Breaking: an implementation of OutboxStore must provide rekey(). Both shipped stores do.

EntityKey::equals() and Mutation::withEntity() come with it — the rename needs both, and neither was expressible from outside.