Merge release6.1.x to mainbranch


git-svn-id: https://svn.tribler.org/abc/branches/mainbranch@31942 001aeff7-3401-0410-a489-f7902fc005dd
Remove print_stack from commitNow

git-svn-id: https://svn.tribler.org/abc/branches/mainbranch@31943 001aeff7-3401-0410-a489-f7902fc005dd
Merge remote-tracking branch 'mainbranch'