Headers set/add/contains timeMillis methods

Motivation:
The new Headers interface contains methods to getTimeMillis but no add/set/contains variants.  These should be added for consistency.

Modifications:
- Add three new methods: addTimeMillis, setTimeMillis, containsTimeMillis to the Headers interface.
- Add a new method to the Headers.ValueConverter interface: T convertTimeMillis(long)
- Bring these new interfaces up the class hierarchy

Result:
All Headers classes have setters/getters for timeMillis.