Initial draft of wasm server integration

[tailhook]

This is as simple as it can get:

1. Has single version of extension without any configuration
2. Relies on wasm server listening on $runstate_dir/.s.wasm_ext
3. Uses unix socket with one connection per request (similarly to SCGI)
4. Uses Pythonic Pickle to serialize request/response

Technically it works, but doesn't make too much sense to integrate right now. Anyone having access to edgedb/edgedb-wasm repo (still private) can play with it though.

[elprans]

Can we add some basic tests here?

[tailhook]

This is generally ready, but I'm not sure if we want to fix GC issue somewhere else.

[fantix]

This is generally ready, but I'm not sure if we want to fix GC issue somewhere else.

#3904 should've fixed most - if not all - of the GC issue here, so that we don't need to override StreamReaderProtocol in this PR. I'm looking into the create_task()s in uvloop.

[tailhook]

Okay, removed that hack.