tmf.core: Introduce ITmfTimestamp#toNanos

tmf.core: Introduce ITmfTimestamp#toNanos

This method can be used to quickly convert an ITmfTimestamp to a
long using nanoseconds. This should reduce code re-use but also
accelerate the fast path as it saves object creation and avoid
errors in copy pasted code chunks.

Change-Id: I402c419fe2d579cf5e7e07b258665adbf8af8f8b
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/67090
Reviewed-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>