make possibly conflicting optimistic cache updates write to each other

[huumn]

My theory as to why #1655 and #1669 occur is that an earlier and conflicting optimistic mutation fails while another optimistic mutation is in progress. When the optimistic mutation fails, it rolls back the cache to its initial state which causes the in-progress optimistic mutation to have its cache updates overwritten.

In the case of #1655, I believe that this occurs when an earlier zap (in a succession of zaps) fails. In the case of #1669, I believe this occurs when a poll is zapped, then voted on.

I've made the relevant cache updates write to each other. It's very hard to reproduce so I'm not sure I got all the relevant cache.modify calls, but I'll continue looking for any cache updates on the optimistic response path in the few places we do it (zaps/boosts/polls) ... it's unclear if there are consequences to doing it on every cache.modify call.

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Update: I've made all the cache.modify calls on items update optimistic caches. This might be unnecessary overhead but afaict it shouldn't cause anything bad to happen, and should prevent optimistic rollbacks from affecting these cache updates.

[huumn]

I'm going to leave those issues open just in case this doesn't fix it.