Drop completed blocked `ChannelMonitorUpdate`s on startup

Drop completed blocked `ChannelMonitorUpdate`s on startup

If a user receives a payment preimage for an outbound payment, the
`PaymentSent` event will block any eventual RAA
`ChannelMonitorUpdate` from the same channel, assuming it comes in
before the event can be processed. If this blocking kicks in, but
the flow eventually completes with the RAA `ChannelMonitorUpdate`
being persisted, but the `ChannelManager` is only persisted prior
to the event being handled, on startup we'll have a fully
up-to-date `ChannelMonitor` but a pending, blocked
`ChannelMonitorUpdate`. When the `PaymentSent` event is replayed
we'll end up trying to apply a redundant `ChannelMonitorUpdate`
which will panic.

See the test added in this commit for an implementation of this
situation.

In this commit we fix this issue by simply dropping blocked
`ChannelMonitorUpdate`s the same as we do pending ones.