Add support for context manager protocol to all samplers

[randomir]

Now all of QPU (DWaveSampler, DWaveCliqueSampler), and hybrid (LeapHybrid*Sampler) samplers support .close() and runtime context use.

Close #91.

Composites will be addressed in a follow-up as per #555.

[codecov]

Codecov Report

Attention: Patch coverage is 79.48718% with 8 lines in your changes missing coverage. Please review.

Project coverage is 85.40%. Comparing base (46efbe5) to head (3b86960).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
dwave/system/testing.py	52.94%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #556      +/-   ##
==========================================
- Coverage   88.79%   85.40%   -3.40%     
==========================================
  Files          21       21              
  Lines        1669     1692      +23     
==========================================
- Hits         1482     1445      -37     
- Misses        187      247      +60     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[arcondello]

LGTM!

[JoelPasvolsky]

LGTM