Factor out the mempool into its own source file

:Release Notes:
Extract the source code of the memory pool into its own file,
create unit tests for it.

:Detailed Notes:
Extract memory pool to parser_memory_pool.
Create unit tests TestParserMemPool.
Change size of the pool.

:Testing Performed:
Unit tests

:QA Notes:

:Issues Addressed:
[BHV-2369] pbnjson: Factor out the mempool into its own source file

Open-webOS-DCO-1.0-Signed-off-by: Andrii Zora <andrii.zora@lge.com>

Change-Id: I478c292522fbc319f7c8f1b14148b1986f78113a
Reviewed-on: https://g2g.palm.com/5203
Reviewed-by: DCO Verification
Reviewed-by: Andrii Zora <andrii.zora@lge.com>
Tested-by: Andrii Zora <andrii.zora@lge.com>
Reviewed-by: Sergiy Kryvonos <sergiy.kryvonos@lge.com>