Remove mock instant

[dfaust]

Fixes a couple of issues:

• Fix notify-debouncer-full by removing mock_instant crate. Trying to use mock_instant across multiple crates was a bad idea. All functions requiring a clock were removed from the notify-types crate and custom time mocking functions were added.
• The crossbeam feature was renamed to crossbeam-channel but that wasn't updated everywhere.
• Add some basic integration tests. These tests are meant to just test if the watchers work at all, not all of their features. That was tried in previous versions of notify, but lead to very flaky tests.
• Update change log.