tmf: CtfTmfEvent improvements

tmf: CtfTmfEvent improvements

- Remove extraneous Javadoc
- Change fTimestamp from a singleton to a proper field, set at
  the constructor.
- Drop unneeded singletons (fReference and fSource)
- Remove getTimestampValue(), call getTimestamp.getValue() instead,
  it is more compatible with ITmfEvent/Timestamp interfaces.

Change-Id: I0522a40f3817bcc34bde48dc90358fb745f7bdbc
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/7648
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bhufmann@gmail.com>