Include an `outbound_payment` flag in `MaybeTimeoutClaimableHTLC`

Include an `outbound_payment` flag in `MaybeTimeoutClaimableHTLC`

When the user is fetching their current balances after forwarding a
payment (before it clears), they'll see a
`MaybePreimageClaimableHTLC` and a `MaybeTimeoutClaimableHTLC` but
if they sum up their balance using
`Balance::claimable_amount_satoshis` neither will be included.

Obviously, exactly one of the two balances should be included - one
of the two resolutions should happen in our favor. This causes our
visible balance to fluctuate up and down by the full value of any
HTLCs we're in the middle of forwarding, which is incredibly
confusing to see. If we want to stop the fluctuations, we need to
pick one of the two balances to include. The obvious candidate is
`MaybeTimeoutClaimableHTLC` as it is the lower of the two, and
represents our balance without the fee we'd receive from the
forward.

Sadly, if we always include it, we'll end up also including any
HTLCs which we've sent but which haven't yet been claimed by their
recipient, which is the wrong behavior.

Luckily, we have access to the `Option<HTLCSource>` while walking
HTLCs, which allows us to add an `outbound_payment` flag to
`MaybeTimeoutClaimableHTLC`. This allows us to only include
forwarded payments in `claimable_amount_satoshis`.

Sadly, even with this in place our balance still fluctuates by the
changes in the commitment transaction fees we have to pay during
forwarding, but addressing that is left for later.