Add DB::Transaction class. Change tests to use new libprivilege API.

Add DB::Transaction class. Change tests to use new libprivilege API.

[Issue#]       SSDWSSP-183
[Bug/Feature]  N/A
[Cause]        N/A
[Solution]     Added class DB::Transaction which opens and closes transaction.
               Added macros DB_BEGIN and DB_END that determine DB::Transaction
               class object lifetime which in turn start and commit
               db transactions.
               Also macros used in tests to reduce test-execution time.
[Verification] Build, install, run tests.

Change-Id: I90b48c27e352fb24e9c57ec7b1bb9005d8f0fd2d