ctf: CtfTmfEvent improvements

ctf: CtfTmfEvent improvements

- Rename getReference() to getChannel().
- Rename getCPU() to getCpu().
- Remove getID(). It was not used by anything, and as was mentioned
  in its Javadoc, it's better to use the event name instead, so it
  is not very useful to expose it.
- Add @NNBD, and related annotations and checks
- Add equals() and hashCode() methods.

Change-Id: I83eb42b01b98895bbdb63a8c6fdba7249f93ec06
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/52121
Reviewed-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>