Rationale: it's easier to reason about the thread lifecycle. The new
ThreadLoop class has the same API as the old SFLThread class, but wraps
an std::thread instead of a pthread, exits on exceptional errors via an
exception and calls its owner's methods via function handles (setup,
process and cleanup)

Refs #47343

Change-Id: Ie1eec81ec53cdc5bb6f00a89a916c5c8f9abeb4f