tmf: Remove the serialization logic from TmfStateValue

tmf: Remove the serialization logic from TmfStateValue

Serializing the state values is the history backend's job (as in,
each backend could do it differently). Move the existing logic
to HTInterval.

This also fixes a very bad bug where the stringsEntrySize of intervals
containing a Long state value would be incorrectly reported as 0.
This would mean that nodes could try inserting a value that wouldn't
fit, which would end up corrupting the interval data.

Change-Id: I6c54ef905b45b06b69ed21ce59c70d2eaa16b7f8
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/12254
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>