Extend DateBone to allow for time values with microseconds

[phorward]

There are several issues that confuse or point out a todo for a DateBone supporting microseconds. There was also a request from a customer project. Therefore this issue is created.

          > > Okay. I was wrong. The datastore client doesn't throw it away, but the core in the `FileBone` does:

viur-core/core/bones/date.py

Line 242 in 19f1351

value = value.replace(microsecond=0)

So I don't understand why we have to do this here again.

FileBone? I dont now why throw it away. I implement the old way. May be it's historical. The datastore support microseconds in the timestamp. https://cloud.google.com/datastore/docs/reference/data/rest/Shared.Types/Value

The reason for setting the microseconds to 0 is, that additional, project-specific program code might rely on a datetime-object which is the exact time to seconds, without microseconds. In the past, I had a use-case where this was a big problem and why it was added. microseconds should only be set when the DateBone is in a yet not existing time-with-microseconds-mode.

I would like to take this PR for now as is. We can later change the behavior to respect microseconds when this is configured so.

Originally posted by @phorward in #785 (comment)