Fix thread-safety (or document its absence)

[xmo-odoo]

Because ptracer uses a context manager, it's easy to use it in a multi-threaded context where the context can be entered and exited from multiple threads concurrently. However it seems to deal rather badly with doing that: because ptracer.context works with an unprotected global (_context), that object can be in a completely inconsistent state because there's a TOCTOU bug on self.enabled.

An easy fix would be to replace enabled with a regular lock (and acquire it in non-blocking, raising if it was not acquired).

It would be nice if ptracer could handle the situation better by having each context "subscribe" to the tracing process, but the asynchronous nature of the reporting likely makes that complicated.

[jparise]

@elprans do you have any initial thoughts on how to approach this?