CA provider's threadAttach() doesn't disconnect an already-attached context

[anjohnson]

If a program creates its own CA context and also happens to use the PVA CA provider, the threadAttach() method's call to

     ca_attach_context(current_context);

will fail silently, and subsequent CA calls inside the provider will fail. Fixing this will probably require some re-engineering of how the context is set to implement an RAII object that saves the CA context of the caller and temporarily instantiates our own, then restores theirs on exit if it was non-zero (similar to a mutex guard object).

Low priority since this isn't likely to be a common thing to want to do.

[anjohnson]

Assume closed by #167