Properly batch Client.delete_many() calls

[shargan]

Essentially the same change as #182 but for delete_many() and its underlying _misc_cmd() method. This includes benchmarks for the delete methods.

Benchmarks for the current codebase with irrelevant lines trimmed:

# 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_delete[pymemcache] 0.296519994736
pymemcache/test/test_benchmark.py::test_bench_delete_multi[pymemcache] 27.9956350327


# 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_delete[pymemcache] 0.2534470558166504
pymemcache/test/test_benchmark.py::test_bench_delete_multi[pymemcache] 25.861117839813232

With this diff applied, delete_multi() is significantly improved:

# 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_delete[pymemcache] 0.310829877853
pymemcache/test/test_benchmark.py::test_bench_delete_multi[pymemcache] 5.40752792358


# 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_delete[pymemcache] 0.30205321311950684
pymemcache/test/test_benchmark.py::test_bench_delete_multi[pymemcache] 6.920046091079712

[cgordon]

This one is going to require more time to review than I have this morning, so I'll try to take a closer look this weekend.

[shargan]

No rush. Thanks for being willing to consider it.

[nichochar]

I looked at this and it looks OK, but I'll let @cgordon take a look and validate. Once he's happy with it, we can merge and cut a release.

Thanks for the change

[nichochar]

I have a slight preference for joining here with the \r\n instead of adding it to each cmd, but it's not a huge deal.

[nichochar]

Thanks for the improved readability!

[cgordon]

Looks great, apologies for the delay, and thank you!