Wake `background-processor` from `ChainMonitor` on new blocks

Wake `background-processor` from `ChainMonitor` on new blocks

When we receive a new block we may generate
`Event::SpendableOutputs` in `ChannelMonitor`s which then need to
be processed by the background processor. While it will do so
eventually when its normal loop goes around, this may cause user
tests to be delayed in finding events, so we should notify the BP
immediately to wake it on new blocks.

We implement that here, unconditionally notifying the
`background-processor` whenever we receive a new block or confirmed
transactions.