Do not hold a global lock while a widget is constructed

[maartenbreddels]

Summary

Element.create_lock is removed: no lock is held while a widget is constructed anymore.
The recording of side-effect widgets is per thread instead.

Why

The lock was one lock for the whole process, held around widget.__init__.
Constructing a widget opens a comm, and in a server (solara) that is a websocket send, which can wait for a client that stopped reading, for example a laptop that went to sleep.
One such client then blocked widget creation for every other user of the server, until TCP gave up.

The lock was added in 2022 (efe7f61, "make thread save") because orphans were detected with a before/after diff of the global widgets dict: "Because we look before and after, we need a lock. A different implementation might avoid this."
The construction hook (20efc14) replaced the diff, but kept the recording in one module level list, so the lock stayed.
With the recording per thread (a typed threading.local), each thread only sees its own constructions, and the lock has nothing left to protect.

It also fixes a bug: a widget that another thread created while a render constructed a widget was recorded as that widget's side effect, and closed together with it.

Tests

In reacton/core_test.py:

• a construction that blocks on one thread does not block widget creation on another thread, and side-effect widgets are still attributed to their own render context;
• a widget created by another thread during a construction is not an orphan;
• nested recordings: the inner one collects its own widgets, and the outer one continues afterwards.

The first two fail on master, the third tests the new helper.

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