[transaction] explicit Transaction cnxset attribute The Transaction object have a lot more attribute that those previously declared in __init__. They are just dynamically assigned in Session code. This changeset explicit the ``cnxset`` and ``mode`` attribute declaration.

[server] rename TransactionData to Transaction Transaction will become a full featured object.

[doc] update Session documentation A major refactoring of Session/Transaction handling is commit. This is a good occasion to improves the documention.

merge 3.16 fix in default

[devtools] fix a couple issues with xvfb-run xvfb-run didn't quite clean up correctly after itself, because: - dash doesn't run EXIT traps on signals - if we don't run the command in the background, a TERM handler doesn't run until the command exits

[devtools/qunit] don't open-code subprocess.Popen.terminate

[devtools/qunit] get rid of unused variable

[server] *init_repository* lookup the database instead of the schema to drop tables (closes #810743) So, tables are dropped even if the schema was changed. :note: if mssql, drop views before tables. :note: because of table dependencies we try a second drop sequence

[book] remove XXX to an old discussion about appengine

[web publish] in case of error, ensure proper http status is set and Content-disposition header is reseted. Closes #2553066