Order ChannelManager lock acquisition

Order ChannelManager lock acquisition

Since last_block_hash is read while updating channels and written when
blocks are connected and disconnected, its lock should be held for the
duration of these operations. Further, since the channel_state lock is
already held during these operations, an ordering of lock acquisition
should be maintained to enforce consistency.

Ordering lock acquisition such that channel_state is held for the
duration of last_block_hash gives a consistent ordering.