Refactor jobject to use GHashTable

:Release Notes:
Use GHashTable to keep key-value pairs of a JSON object
Use strong djb2 string hash function
Reduce jobject iterators to two functions
Fix automatic tests
Advance version to 2.0.0

:Detailed Notes:
The change is meant to improve the code clarity first of all.
The idea comes from cjson, which uses hash table with double-
linked elements for sequential iteration. JSON doesn't require
any particular order for object members, thus, GHashTableIter
will serve well.

Supposedly, the refactored code will perform better on
JSON object access, and sequential iteration. The performance
of parsing is roughly the same as of the previous implementation.

:Testing Performed:
Built successfully.
Ran automatic tests successfully.
Smoke tested on QEMU and on a real device.

:QA Notes:
The version of the component is 2.0.0 because of incompatible changes
to the JSON object iterators.

:Issues Addressed:
[GF-6969] libpbnjson: Refactor jobject to use GHashTable instead of m_buckets

Open-webOS-DCO-1.0-Signed-off-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>

Change-Id: I52fa62a9d6056353bb36945d9b5227b04429071d
Reviewed-on: https://g2g.palm.com/2828
Reviewed-by: Build Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>