xml analysis: internalize strings.

xml analysis: internalize strings.

Stings are currently taking a major amount of memory. This patch reduces
the memory usage by making the strings internal (cached) so duplicates
are no longer copied in memory.

Change-Id: Id686ea8c0648ddf2a7d041b6975ffbab6607167c
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/70908
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Hudson CI