Change LND unreachable case from `WaitingUnlock` -> `Disconnected`

[kilrau]

Off-shoot from https://github.com/ExchangeUnion/xud/pull/2004#issuecomment-737058817 :

The error 24/11/2020 09:32:03.099 [LND-BTC] error: AssertionError [ERR_ASSERTION]: awaitWalletInit should not be called from a status besides Initialized or Unlocked is unrelated to this PR - I'll look into it separately. It's a bit of an edge case but I can reproduce it by setting up a new node with an enabled lnd in the config that is unreachable. It's setting the state of that lnd to WaitingUnlock when it should really be disconnected, so it tries to unlock it anyway and hits that exception.