Fix JSON schema validation in multithreaded environment

:Release Notes:
Reference counting removed from static validator inctances

:Detailed Notes:

:Testing Performed:
Unit tests with valgrind

:QA Notes:

:Issues Addressed:
[GF-42809] pbnjson: Fix reference counting of generic_validator (and
           alike) in multithreaded environment

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

Change-Id: I5d2f064bf1f63d6975cc46c4991d4d764f37ef94
Reviewed-on: https://g2g.palm.com/3694
Reviewed-by: Build Verification
Reviewed-by: Mykola Kryvonos <mykola.kryvonos@lge.com>
Tested-by: Mykola Kryvonos <mykola.kryvonos@lge.com>
Reviewed-by: Denys Sokolovskyi <denys.sokolovskyi@lge.com>
Tested-by: Denys Sokolovskyi <denys.sokolovskyi@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>