[Ur] Code review request

[Sergey Mironov]

Hi. I've discovered a mysterious bug in my callback library during the
stress-testing. Probably, it is a multithreading issue. In summary,
the library starts several callback threads during initialization,
each thread creates the `uw_context', opens database connection and
waits for incoming callback requests.  The rest of the library
executes the server-side tasks and sends results (stdout, exit code,
etc) to the threads. All the problems below are demonstrated by the
Demo.exe application (demo/ folder of the project sources).

The facts are:

- When I run my application with only one callback thread, everything works fine
- Increasing the number of callback threads up to 10 results in 95%
chance of Segfault during init, but only if no debugger attached.
Executing the Demo.exe via GDB doesn't show any crashes.
- Increasing the number of main urweb threads (-t N argument) doesn't
affect the situation
- Sometimes the Demo.exe starts successfully (~ 5% chance with 10
callback threads). In this case feeding it with tasks results in
random 'Serialization failure's reported from the callback threads.

Since gdb can't help me, I feel I'm in trouble. Could you please
review the code and point me to the potential pitfalls?
System-specific things seem to work, the problem is likely in the
urweb-specific part of the library. Do I call uw_init correctly? Is it
OK to call uw_begin_init like I do?

The thread code is in the `devel' branch of the urweb-callback repo,
CallbackFFI.cpp, struct notifiers (line 377)

https://github.com/grwlf/urweb-callback/blob/devel/CallbackFFI.cpp#L377

Thanks in advance,
Sergey