feat: add if*generation*Match support, pt1

[IlyaFaer]

Add if_generation_match, if_generation_not_match, if_metageneration_match and if_metageneration_not_match arguments into methods:

Client(): lookup_bucket(), get_bucket()

Bucket(): get_blob(), delete_blob(), delete(), exists(), patch(), reload(), update()

Blob(): delete(), download_to_file(), download_to_filename(), download_as_string(), rewrite(), update_storage_class(), exists(), patch(), reload(), update(), reload()

Towards #127

[frankyn]

Client-side checks for if_*generation checks should not be added for this case.

Please remove use of _raise_for_more_than_one_none and let the service handle those errors.

[andrewsg]

Looks like the system tests are in part 2. So that we don't merge an incomplete change, please go ahead and merge part 2 into this PR and close that one. It will be a large PR but that's better than risking merging one and having the other delayed.

[IlyaFaer]

In fact, parts 1 and 2 are completely separated from each other, I've just tried to make PRs easier to read. Anyway after erasing the checks changes become much more shorter, so I suppose we can merge part 2 into this one.

[IlyaFaer]

Lint session failed, but the problem is not related to this PR:

Running session lint
Creating virtual environment (virtualenv) using python3.8 in .nox/lint
pip install flake8 black==19.10b0
black --check docs google tests noxfile.py setup.py
All done! ✨ 🍰 ✨
35 files would be left unchanged.
flake8 google tests
google/cloud/storage/_signing.py:156:13: F523 '...'.format(...) has unused arguments at position(s): 0
Command flake8 google tests failed with exit code 1
Session lint failed.

[andrewsg]

Thanks Ilya. This looks great, only thing to note is that it looks like the metageneration match could use a system test. I don't think there's utility in making a system test for each modified function, but adding a system test for the metageneration match in addition to the generation might be a sweet spot for effort/reward. Should be good to go after that's in.

[IlyaFaer]

I've added more system tests, and re-reviewed it all once again - fixed a couple of nits.

[frankyn]

One nit, but otherwise LGTM. Pending @andrewsg's comments.

[frankyn]

LGTM, pending @andrewsg's approval.

[andrewsg]

Thanks!