[schema] store default attribute values in a Bytes field, allowing python objects as default values
Notes:
* Binary objects grow two methods
* we depend on a newer yams version
* the code is quite simplified as a result of the whole change
* a very old and nasty bug where bool(default values) evaluating to
False not being properly migrated is killed
Closes #2414591.
[source/native] fix sqlserver pattern matcher for unique constraints (closes #3227593)
The matcher is used to parse the low-level error message and extract
meaningful information for the end users.
If broken, the users see an alarming and bewildering message.
A full & definitive fix is scheduled for CubicWeb 3.18.