Refined semantics for Settings thread-safety

[okhat]

1. Only one unique thread (which can be any thread!) can call dspy.configure.
2. It affects a global state, visible to all. As a result, user threads work, but they shouldn't be mixed with concurrent changes to dspy.configure from the "main" thread. (In the future, one can add warnings for this, if there are near-in-time accesses to the configuration followed by .configure calls.)
3. Any thread can use dspy.context. It propagates to child threads created with DSPy primitives: Parallel, asyncify, etc.