Make DWaveSampler picklable

[pgawron]

DWaveSampler is not picklable. It should be in order to integrate for example with sklearn.

In [1]: from dwave.system.samplers import DWaveSampler

In [2]: import pickle

In [3]: annealer = DWaveSampler()

In [4]: pickle.dumps(annealer)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-bef1022bace0> in <module>()
----> 1 pickle.dumps(annealer)

TypeError: can't pickle _thread.lock objects

[randomir]

This basically translates to making dwave.cloud.Client picklable (in dwave-cloud-client package). Currently it isn't by default, because thread pools and mutexes would have to be recreated manually.

Can you show here the exact example that requires the DWaveSampler to be picklable? Thanks.