tmf: Add equals/hashCode to TmfConstantTransform

tmf: Add equals/hashCode to TmfConstantTransform

Without that, two equivalent transforms would be considered
different, because the object is serialized every time, causing
it to become a new instance every time a trace is re-opened.

Change-Id: I4f1dcbf503032329cdce7ac5c0ef351545e392e8
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/82779
Reviewed-by: Hudson CI
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>