Separating _patch_properties and PATCH on Bucket.

[dhermes]

@tseaver there is plenty to discuss here and I will enumerate.

In the immediately relevant category is my uses of self.patch() in several of the methods. I was unclear if we think they should be setters or actual API requests.

Relates to #632.

[dhermes]

Some other things to worry about

1. In the future where connection is not bound to Bucket

    def patch(self, connection=None):
        self._properties = connection.api_request(...

This would be a 204 on a Batch and do very much the wrong thing
2. Most getters use _PropertyMixin.properties, which is always an API request (See #688)
3. Blob.metadata (setter) is the only use of _patch_properties there in Blob. It seems _scalar_property was the biggest user. (See #690, it changes _patch_properties everywhere.)
4. This may break _PropertyBatch as it replaces _patch_properties on enter and puts it back on exit. (See #690, it changes __exit__.)

[coveralls]

Coverage remained the same at 100.0% when pulling cdd6694 on dhermes:patch-bucket-imperative into 51e286f on GoogleCloudPlatform:master.

[dhermes]

@tseaver PTAL

[tseaver]

LGTM.