Fix session persistence handlers

[gaearon]

There is a regression in #4857 — although it appears to be setting up the session persistence handler, that handler (stored in a ref) is not being read. Additionally, it is a little fragile that merely calling createAgent* modifies a ref since create* functions were intended to be stateless. Otherwise it gets very difficult to reason about race conditions.

In this PR, I remove the ref and move the persistence handler to be scoped per-agent like before. The underlying BskyAgent signature changed so that the persistence handler has to be supplied during creation. However, we don't always know the did during its creation (e.g. we wouldn't know the did of an agent whose createAccount call is yet to follow). So we want late binding for the session handler. To do that, I store it in a local variable and swap it out when ready.

I ended up removing prepareAgent handler because it was obscuring what's actually happening. This adds a bit of duplication but should be clearer.

Test Plan

Have not explicitly tested but @haileyok knows a few bugs that this should fix.

[render]

Your Render PR Server URL is https://social-app-pr-4927.onrender.com.

Follow its progress at https://dashboard.render.com/web/srv-cqtbsba3esus73e597r0.

[gaearon]

Note I am no longer cleaning up the persistence handler when the current agent gets swapped. I think this should be fine. It didn't used to be cleaned up in the past either, I only added this at some intermediate stage for peace of mind. But we already have a condition here that prevents inactive agents from clearing persisted data. For inactive agents, refreshedAccount should be undefined because we already set prevAgent.sessionManager.session = undefined here. And even if somehow it had a session object, the reducer always matches events with state by the did so it won't update the wrong thing.

[github-actions]

Old size	New size	Diff
7.09 MB	7.09 MB	120 B (0.00%)

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[gaearon]

let's go with #4928