[fuzz] Expand chanmon_consistency to do single message delivery

While trying to debug the issue ultimately tracked down to a
`PeerHandler` locking bug in #891, the ability to deliver only
individual messages at a time in chanmon_consistency looked
important. Specifically, it initially appeared there may be a race
when an update_add_htlc was delivered, then a node sent a payment,
and only after that, the corresponding commitment-signed was
delivered.

This commit adds such an ability, greatly expanding the potential
for chanmon_consistency to identify channel state machine bugs.

Author: TheBlueMatt