Persistency: store special double values

Json library handles infinity and NaN as null. During deserialization,
we expect a double and those files were marked as invalid and removed.
To prevent this behavior and preserve all double values, LabeledTuple
'to_json' and 'from_json' functions were improved to accommodate those
special values by using string literals.

Testing done:
- UTs added for double<=>json conversion.
- UTs passing.

Signed-off-by: Szymon Dompke <szymon.dompke@intel.com>
Change-Id: I9193df29cce1db28cda3c895d117d9f3bfca2c24
diff --git a/tests/meson.build b/tests/meson.build
index ce86dd3..29aaeb0 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -40,6 +40,7 @@
             'src/test_detached_timer.cpp',
             'src/test_discrete_threshold.cpp',
             'src/test_ensure.cpp',
+            'src/test_labeled_tuple.cpp',
             'src/test_make_id_name.cpp',
             'src/test_metric.cpp',
             'src/test_numeric_threshold.cpp',