Invalid ref count during persist

[arv]

https://discord.com/channels/830183651022471199/1095424613284065330/1095424613284065330

We are hitting this assert:

const oldCount = this._refCounts.get(refHash);
...
assert(oldCount > 0);

This is in some old code dealing with the in memory cache and the error happens during persist.

We've made significant change to both the on memory cache and to persist so it is possible this assert no longer fails.

[arv]

Most likely fixed by rocicorp/mono@4378df2f