Simplify StackContext logic and behavior.

Simplify StackContext logic and behavior.

Previously, there was an optimization to avoid recreating contexts
that were already on the stack.  This optimization rarely mattered
(especially given the movement to run all callbacks through
IOLoop.add_callback), and sometimes caused surprising behavior as
other exception handlers could be inserted between stack contexts that
expected to be adjacent to each other or to the wrapped function.  Now
each wrapped function recreates its full stack of contexts, even if
some of those contexts were already on the stack.

This change allows a use of NullContext in testing.py to be
removed.