This exposes the previous default behavior at the `tvix-store daemon`
subcommand.
It also adds a `tvix-store import` command, which will ingest a given
path into the store.
Change-Id: Ide14f1d409b9364e7f98090690c744326486e470
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8166
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>