[Ur] Code review request

[Sergey Mironov]

Thanks! I'll pull your updates soon.

> "Serialization failure" is not a fatal error message.  In fact, you should
> expect to see it regularly in applications with high concurrency and many
> SQL writes.  It just means that transactions are being restarted to maintain
> the ACID semantics.

Fine, I've expected this. And I see you called rollback in case of
unsuccessful commit. That actually was my next question :)

Here is another small thing I've just remembered about: I saw uw_error
calls in the body of uw_register_transactional(). Theoretically, this
is not good since FFI programmers may use uw_register_transactional
for registering various free/delete calls (I do). Raising an error
from there would lead to memory leak in the FFI code this case.
Probably, uw_register_transactional() should return an error code
instead. This would give programmer a chance to free their resources
and _than_ raise an error with uw_error.

Regards,
Sergey