Properly batch Client.set_many() calls

[shargan]

pymemcache.client.base.Client.set_many() currently loops over its values dictionary and sends each key-value pair separately rather. This diff drops them in bulk into the fewest TCP packets in which they'll fit. If the overall approach meets your approval, I'll send a separate PR to do the same for delete_many().

Using the expanded benchmarks from #180 and snipping output down to just the relevant lines, the current codebase yields these results:

# python2.7
$ pytest --verbose --capture=no --no-cov -m benchmark pymemcache/test --keys 100 --count 10000
========================================== test session starts ===========================================
platform darwin -- Python 2.7.15, pytest-3.7.4, py-1.6.0, pluggy-0.7.1 -- /Users/shargan/repos/pymemcache/.venv/bin/python

pymemcache/test/test_benchmark.py::test_bench_set[pymemcache] 0.449674129486
pymemcache/test/test_benchmark.py::test_bench_set_multi[pymemcache] 36.8027918339


# python3.6
$ pytest --verbose --capture=no --no-cov -m benchmark pymemcache/test --keys 100 --count 10000
========================================== test session starts ===========================================
platform darwin -- Python 3.6.6, pytest-3.7.4, py-1.6.0, pluggy-0.7.1 -- /Users/shargan/repos/pymemcache/.venv36/bin/python3.6

pymemcache/test/test_benchmark.py::test_bench_set[pymemcache] 0.3536701202392578
pymemcache/test/test_benchmark.py::test_bench_set_multi[pymemcache] 35.68993163108826

With this diff applied, the results show increased set_multi throughput:

# python2.7
$ pytest --verbose --capture=no --no-cov -m benchmark pymemcache/test --keys 100 --count 10000
========================================== test session starts ===========================================
platform darwin -- Python 2.7.15, pytest-3.7.4, py-1.6.0, pluggy-0.7.1 -- /Users/shargan/repos/pymemcache/.venv/bin/python

pymemcache/test/test_benchmark.py::test_bench_set[pymemcache] 0.389505147934
pymemcache/test/test_benchmark.py::test_bench_set_multi[pymemcache] 10.9233689308


# python3.6
$ pytest --verbose --capture=no --no-cov -m benchmark pymemcache/test --keys 100 --count 10000
========================================== test session starts ===========================================
platform darwin -- Python 3.6.6, pytest-3.7.4, py-1.6.0, pluggy-0.7.1 -- /Users/shargan/repos/pymemcache/.venv36/bin/python3.6

pymemcache/test/test_benchmark.py::test_bench_set[pymemcache] 0.38876795768737793
pymemcache/test/test_benchmark.py::test_bench_set_multi[pymemcache] 11.447095155715942

This has the added side effect of allowing future revisions to add X_many() sister methods to any of the other storage commands.

[cgordon]

The code changes look good, and I'm excited to have that ancient TODO done. It looks like Travis is failing due to a few simple formatting issues, so once those are fixed I'll merge. Thanks!

[shargan]

Ah, missed those somehow. Thanks, flake8 complaints are resolved.