Hold ChannelManager locks independently

Hold ChannelManager locks independently

ChannelManager reads channel_state and last_block_hash while processing
funding_created and funding_signed messages. It writes these while
processing block_connected and block_disconnected events. To avoid any
potential deadlocks, have each site hold these locks independent of one
another and in a consistent order.

Additionally, use a RwLock instead of Mutex for last_block_hash since
exclusive access is not needed in funding_created / funding_signed and
cannot be guaranteed in block_connected / block_disconnected because of
the reads in the former.