Context callback changes and device mutex removal

[maloel]

This is an API-breaking change:

• a context is created
• rs2_set_devices_changed_callback is called
• the context is destroyed with rs2_delete_context

Before: as long as the internal context object was held alive (by a device etc.), the callback was still callable, possibly even when unintended (requiring the user to set an "empty" callback as a workaround)

Now: when delete-context is called, the callback is unsubscribed and no longer happens. I.e., you now have to hold the context if you want callbacks from it. This is the correct behavior.

Changes:

• simplify context callback handling; only a single function remains, on_device_changes()
• make use of rsutils::signal (we handle multiple callbacks), and remove mutex in context
• simpler & more robust device::is_valid() implementation, without a mutex around it

Everything together solves the deadlock in #11933. A test will be added separately to regressions.

Tracked on [RSDSO-19304]

[Nir-Az]

Not sure the comment is needed.
Tomorow if we add another used and it compiles, we will need to update this comment?
Will not happen :)

[maloel]

I find it helps -- at least for this version of the code.
But you're right, it likely won't get updated.
Hopefully I'm getting the context to be focused-enough so that it'll never have to be updated again...

[Nir-Az]

LGTM