patch reconnect deadlock

[jackyzha0]

Why

deadlock in the server-side reconnect case:

1. establish handshake acquires session lock: https://github.com/replit/river-python/blob/main/src/replit_river/server_transport.py#L231
2. if there is a session mismatch, we close the old session: https://github.com/replit/river-python/blob/main/src/replit_river/server_transport.py#L272
3. session.close calls _close_session_callback https://github.com/replit/river-python/blob/main/src/replit_river/session.py#L562 which is bound to transport._delete_session
4. _delete_session also tries to acquire the session lock https://github.com/replit/river-python/blob/main/src/replit_river/transport.py#L44

What changed

split up critical sections in server transport similar to client side

1. make _get_session acquire lock, dont need to lock for creating the session until we call _set_session
2. close session also acquires its own lock

Test plan

added a test

Notes

I have a draft of a more in-depth approach on this branch which uses a lock-ownership-transfer based approach that should catch it more generically but ran into ownership problems lol

Seeing as we are planning on migrating chat service to Node anyways so we only have one River server implementation, we hopefully don't have to maintain this surface for much longer 🤞

[lhchavez]

for safety, one property that the old code had that this one doesn't is that there could be two concurrent calls to this function (with the same id?) and then we would close the old session twice and leak one of the new sessions!

to prevent that what we can do is to rename _set_session to _set_session_locked (and add a docstring comment that it needs the session_lock to operate). and have _set_session_locked detect this mid-air collision and ... do something with the preexisting one (close it???)

with that out of the way, this relaxing of the locks' critical sections seems to be safe, because the Session by itself is safe to create twice (as long as we close it, otherwise we leak tasks!!!)

[lhchavez]

lol dupes

[blast-hardcheese]

@jackyzha0 I applied this delta overtop #147, available at jacky-deadlock-patch, so we can keep moving