Refactor forward_htlcs to return whether to push a forward event

Refactor forward_htlcs to return whether to push a forward event

When decoding pending `update_add_htlc` onions, we may need to forward
HTLCs using `ChannelManager::forward_htlcs`. This may end up queueing a
`PendingHTLCsForwardable` event, but we're only decoding these pending
onions as a result of handling a `PendingHTLCsForwardable`, so we
shouldn't have to queue another one and wait for it to be handled. By
having a `forward_htlcs` variant that does not push the forward event,
we can ignore the forward event push when forwarding HTLCs which we just
decoded the onion for.