Begin transactions explicitely in Job class.

Begin transactions explicitely in Job class.

This reverts commit dfbf524d3561e3fc54a9a072fda173435acc2e14 which
was "Removed unmatched commits in Job class".

The original change was reverted n the light of the recent discussion about the DBO_TRX flag
and the intended use and bahavior of begin and commit.

See http://www.gossamer-threads.com/lists/wiki/wikitech/300087 for Tim's explanation of DBO_TRX.

This change now implements a new, alternative solution to the original problem of unmatched
calls to commit:

The Job class originally relied on implicitely started transactions. Introducing
explicit calls to begin() avoids warnings while running tests, and
causes transaxctiosn to be used also in cli (maintenance) mode.

Change-Id: I6ecb8faad06449331a79b81860fe64624d3694d4