This commit adds a new method, `NeedWaitAttempts`, to properly decide
whether we need to wait for the outcome of htlc attempts based on the
payment's current state.
This commit moves the struct `paymentState` used in `routing` into
`channeldb` and replaces it with `MPPaymentState`. In the following
commit we'd see the benefit, that we don't need to pass variables back
and forth between the two packages. More importantly, this state is put
closer to its origin, and is strictly updated whenever a payment is read
from disk. This approach is less error-prone comparing to the previous
one, which both the `payment` and `paymentState` need to be updated at
the same time to make sure the data stay consistant in a parallel
environment.
This commit adds a new method, `Registrable`, to help decide whether
adding new HTLCs to a given payment is allowed. A new unit test,
`TestRegistrable` is also added to test it.
