Add devices cache, extend test

:Release Notes:
Add devices cache; bugfix; existing tests improvements

:Detailed Notes:
- add devices cache with purpose to speedup id search requests
- devices cache is now integrated to MojDbShardEngine
- add convert functions between ShardInfo and MojObject (code reduce)
- extended test MojDbShardManagerTest
- fixed bug within update()
- fixed test MojDbNewIdTest
- add descriptions for all functions of MojDbShardEngine

:Testing Performed:
tested on build

:QA Notes:

:Issues Addressed:

Open-webOS-DCO-1.0-Signed-off-by: Igor Kuksyuk <ikuksyuk@luxoft.com>

Change-Id: I2f704750f558cb6aa9ed4f784c4febf160de961b
Reviewed-on: https://g2g.palm.com/3333
Reviewed-by: Build Verification
Reviewed-by: Roger Stringer <roger.stringer@lge.com>
Tested-by: Roger Stringer <roger.stringer@lge.com>