Process messages from peers in parallel in PeerManager.

This adds the required locking to process messages from different
peers simultaneously in `PeerManager`. Note that channel messages
are still processed under a global lock in `ChannelManager`, and
most work is still processed under a global lock in gossip message
handling, but parallelizing message deserialization and message
decryption is somewhat helpful.

Further, the next commit will somewhat parallelize the CPU-intensive
parts of gossip message handling.