tmf: Refactor TMF statistics

tmf: Refactor TMF statistics

Merge TmfBaseStatisticsTree into AbsTmfStatisticsTree. The abstract
version required both total and per-event-type statistics values
anyway, so there was no need to have a separate class that basically
implemented the required stuff.

Also renamed TmfStatisticsTreeNodeFactory to TmfStatisticsManager,
since it's more a manager for the complete stats trees.

Change-Id: If9942beb14ee0d978722d97e6bdfeafb693c9b66
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/8265
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>