lttng.ust: Introduce a cache of the calls to addr2line

lttng.ust: Introduce a cache of the calls to addr2line

Since calling an external process is relatively costly, we can
cache the results of the calls so future lookups can reuse the
information.

Change-Id: I5d84be78d45b893ea5942bdd5d2459faf4574af5
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/69972
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>