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* fix: minimize time holding wantlist lock

Instead of holding the lock the entire time we prepare a message, hold the lock
while we retrieve the wantlist entries, process the entries without the lock,
retake the lock, then mark entries as sent.

This means:

1. We never sort entries while holding the lock.
2. We allocate exactly three times while holding the lock (once per entry list).

* fix: address code review

This makes reading the docs much easier as it's clear what's "private" and
what's not.

fixes #238