19686: api constructor returns ThreadSafeApiCache

19686: api constructor returns ThreadSafeApiCache

This is an API-compatible wrapper object that provides thread
safety. Returning this from api() helps keep users out of
trouble.

The changes to ThreadSafeApiCache are required to keep it API-compatible
with the original and keep tests passing. Assignments to the request_id
attribute need to be used for all future requests. It's a little unclear
if this is an intended API or just test scaffolding, but it's not too
difficult to keep working so I just did that.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>