fuzz: model chanmon persistence in harness

Replace the chanmon consistency harness' Watch wrapper with a Persist
implementation backed by HarnessPersister. Monitor writes now flow
through the real ChainMonitor persistence hooks.

Track restart candidates separately from monitor completion callbacks. A
monitor can stop being a valid reload candidate once a newer baseline is
durable, while its callback may still be needed to unblock the live
ChainMonitor.

On reload, choose the durable baseline, first pending snapshot, or last
pending snapshot. Startup monitor registration completes immediately
before the configured persistence style is restored.

Author: joostjager