Update `ChannelUpdate::timestamp` when channels are dis-/en-abled

Update `ChannelUpdate::timestamp` when channels are dis-/en-abled

We update the `Channel::update_time_counter` field (which is copied
into `ChannelUpdate::timestamp`) only when the channel is
initialized or closes, and when a new block is connected. However,
if a peer disconnects or reconnects, we may wish to generate
`ChannelUpdate` updates in between new blocks. In such a case, we
need to make sure the `timestamp` field is newer than any previous
updates' `timestamp` fields, which we do here by simply
incrementing it when the channel status is changed.

As a side effect of this we have to update
`test_background_processor` to ensure it eventually succeeds even
if the serialization of the `ChannelManager` changes after the test
begins.