Change Tag to hold a String instead of Cow<str>

[morrisonlevi]

What does this PR do?

Change Tag to hold a String instead of Cow<str>.

I also implemented AsRef<str> for Tag.

Motivation

This originated from a lint on nightly which didn't like Cow::to_owned
being called, and instead callers should use Cow::clone or
Cow::into_owned. However, while inspecting the code with fresh eyes, we
saw that we're always holding a Cow::Owned variant, so just hold the
String instead.

Simultaneously, I got a crash report in Rust for the PHP profiler. I
don't support forking, and they forked. Anyway, for this particular
crash it's not much harder to support fork (well, it leaks
technically but works) than to disable everything on a fork. As part of
supporting fork, I need to fixup the process_id tag to have the new
pid after a fork, and Tag::as_ref will help with that.

Additional Notes

I also noticed that we were fully qualifying two usages of DateTime and
Utc even though we're publicly using those, so I shortened them.

How to test the change?

If your code is calling Tag::to_owned then it will break. It should be
reworked into using a Tag::clone or call .to_string on it or similar.

[r1viollet]

LGTM