Queue BackgroundEvent to force close channels upon ChannelManager::read

Queue BackgroundEvent to force close channels upon ChannelManager::read

This results in a new, potentially redundant, `ChannelMonitorUpdate`
that must be applied to `ChannelMonitor`s to broadcast the holder's
latest commitment transaction.

This is a behavior change for anchor channels since their commitments
may require additional fees to be attached through a child anchor
transaction. Recall that anchor transactions are only generated by the
event consumer after processing a `BumpTransactionEvent::ChannelClose`
event, which is yielded after applying a
`ChannelMonitorUpdateStep::ChannelForceClosed` monitor update. Assuming
the node operator is not watching the mempool to generate these anchor
transactions without LDK, an anchor channel which we had to fail when
deserializing our `ChannelManager` would have its commitment transaction
broadcast by itself, potentially exposing the node operator to loss of
funds if the commitment transaction's fee is not enough to be accepted
into the network's mempools.