Reduce memory usage of JSON values

:Release Notes:
Refactor jvalue to reduce memory usage of JSON values

:Detailed Notes:
Previously, a union was used to keep distinct JSON types in a jvalue.
With union size equal to the largest member (the champion was jarray),
this is an obvious overhead, especially considering that all object keys
are JSON strings.
This change refactors the jvalue struct to be embedded into other JSON
types, thus allowing its size to fit memory requirements of specific
types closer.

:Testing Performed:
Build succeeded. All unit tests passed. Also added bash script to
measure peak memory usage of GTest unit tests to verify reduction of
memory usage.

:QA Notes:

:Issue Addressed:
[GF-8743] pbnjson: Improve memory usage of jvalue

Open-webOS-DCO-1.0-Signed-off-by: Mykola Kryvonos <mykola.kryvonos@lge.com>

Change-Id: I235457632abc6b84a017a5b96cf78cd3f45eb1cf
Reviewed-on: https://g2g.palm.com/2939
Reviewed-by: Build Verification
Reviewed-by: Mykola Kryvonos <mykola.kryvonos@lge.com>
Tested-by: Mykola Kryvonos <mykola.kryvonos@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>