ss: cache TmfStateValue String creation

ss: cache TmfStateValue String creation

This patch adds a direct mapped cache to the TmfStateValue#newValueString
method. This method is used heavily in the Lttng Kernel State Provider.

Using KernelAnalysisBenchmark, this patch increases the performance by 5-10%

Change-Id: Ib0bece8eb606186086cbb4843944e438cf5fbf98
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/67455
Reviewed-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Tested-by: Alexandre Montplaisir <alexmonthy@efficios.com>