asset wallet: introduce coin locking and coin selection mutex

[guggero]

The asset wallet introduced in #244 does not allow a coin to be marked as being used (=locking), which needs to be done at the database layer.
Once we can mark asset coins as being used/locked, we also need to make sure that access to coin selection and the locking APIs are covered by a single coin selection mutex similar to how it's done in lnd.

[Roasbeef]

May also want to have a bit that says if an output has been fully spent or not. Either that, or we need to delete it.

[dstadulis]

related issue #374