This commit introduces more granular statuses to better determine a
payment's current state. Based on whether there are inflight HTLCs, the
state of each of the HTLCs, and whether the payment is failed, a total
of 5 states are derived, which can give a finer control over what action
to take based on a given state.
Also, `fetchPayment` now uses `decidePaymentStatus`. After applying the
new function, the returned payment status would be different,
```
| inflight | settled | failed | reason | previous -> now |
|:--------:|:-------:|:------:|:------:|:---------------------------------:|
| true | true | true | yes | StatusInFlight(unchanged) |
| true | true | true | no | StatusInFlight(unchanged) |
| true | true | false | yes | StatusInFlight(unchanged) |
| true | true | false | no | StatusInFlight(unchanged) |
| true | false | true | yes | StatusInFlight(unchanged) |
| true | false | true | no | StatusInFlight(unchanged) |
| true | false | false | yes | StatusInFlight(unchanged) |
| true | false | false | no | StatusInFlight(unchanged) |
| false | true | true | yes | StatusSucceeded(unchanged) |
| false | true | true | no | StatusSucceeded(unchanged) |
| false | true | false | yes | StatusSucceeded(unchanged) |
| false | true | false | no | StatusSucceeded(unchanged) |
| false | false | true | yes | StatusFailed(unchanged) |
| false | false | true | no | StatusInFlight(unchanged) |
| false | false | false | yes | StatusFailed(unchanged) |
| false | false | false | no | StatusInFlight -> StatusInitiated|
```
This commit adds a new payment status, `StatusInitiated`, to properly
represent the state where a payment is newly created without attempting
any HTLCs. Since the `PaymentStatus` is a memory representation of a
given payment's status, the enum used for the statuses are directly
updated.
This commit changes `fetchPaymentStatus` to return an error when the
payment creation info bucket cannot be found. Instead of using the
ambiguous status `StatusUnknown`, we tell the callsite explictly that
there's no such payment. This also means the vague `StatusUnknown` can
now be removed as it has multiple meanings.
