[transaction] initialize transaction data and state related attribute. There is not reason to not explicitly declare and initialize attribute in Transaction.__init__. This change requires the introduction of a Transaction.clear() method to get ride of some wicked ``AttributeError`` based logic.

[transaction] handle ``mode`` default value in Transaction The transaction mode is now explicitly passed at creation time and always read from the Transaction object. Note that there is a slight behavior change. The transaction mode is now set at the creation of the transaction. Changes made to the default value have no longer any effect on existing transaction.

[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