Fix nested schema context thread-safety

[lafrech]

Fixes #1617.
Closes #1791.

This fixes the thread-safety issue but might be a breaking change for people relying on the update: parent schema updates nested schema context. See #1791.

[lafrech]

The alternative, to avoid breaking the update case, would be to document it as not thread-safe and invite the users to use a class rather than an instance, as this is thread-safe (but does not allow context update from parent, only replacement, and doesn't allow to use modifiers in the nested schema init).

Then in 4.0, drop the update case if this is acceptable. And perhaps implement #1826.

The update case allows users to specify a context at both parent and nested level. In practice, I'm not sure this is worth supporting but I could be proven wrong. Besides, the design smells a bit, as the update flattens parent and nested contexts without namespacing nested context. I hope I'm making sense.

[lafrech]

See discussion in #1791.

It might be better not to merge this (breaking change) and instead perhaps raise a warning about the thread-safety issue.