Improve memory allocations during validation against JSON schema

:Release Notes:
Allocate memory during schema validation from GSlice to improve performance.

:Detailed Notes:
Slice allocator allows very easy and fast allocation of objects of
equal sizes. Some validators must maintain their context between
validation events in the stack, combining validators must create
sub-states on initialization. So slice allocation is a natural remedy
in these cases.

:Testing Performed:
Unit tests with valgring

:QA Notes:

:Issues Addressed:
[GF-38338] pbnjson: Measure and tune schema validation performance

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

Change-Id: I2fb78431c7e7f35ff94d1fe213b2ec5bd3d027d6
Reviewed-on: https://g2g.palm.com/3670
Reviewed-by: Build Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>