have to close client threads

[kumagaimasahito]

I faced a problem that 'You can't create a new thread'.
The library 'dwave-system' have client.close(), but DWaveSampler didn't close threads.
Sorry, but I think you may have to add this pull request.

[arcondello]

Hi @kumagaimasahito, I think it would be better to implement this as a close method. The reason is that it is allowed (though poor practice) to do

sampler = DWaveSampler()
client = sampler.client
del sampler

Which would then (perhaps unexpectedly) close the client. I think close would be a lot more explicit (and close an existing issue #77)

See also #91, dwavesystems/dwave-cloud-client#217