Bring back autotest with v2 sync API

[tomasMizera]

Autotests can now compile, run without crashing and mostly succeed. There are two more failing tests that need attention (one because we now consider "already on the latest version" a success and another of unknown reason).

Totals: 59 passed, 2 failed, 0 skipped, 0 blacklisted, 174374ms
********* Finished testing of TestMerginApi *********

The problems were of course not only in autotests - the fixes several problems in the code:

• missing exit path when there is nothing to push
• sync manager kept associating syncs that were not started via it, this caused duplicated connections and extra, undesired, runs of the sync loop

I commented out the test for push cancel as the logic is not implemented yet.

The PR can be reviewed and merged. The other two tests might need some more work.