Bot the version token plugin and the related UDFs are using a common
global data structure.
This structure was disposed of at UNINSTALL PLUGIN and a flag was set
but between disposing of the structure and setting the flag there was
a raise window if some other thread calls the UDF afgain.
Also the global rw lock protecting the common structure was never
disposed of.

Fixed by introducing a check for the global structure right inside the
lock.
Added a C++ class whose destructor releases the RW lock when the
shared object is unloaded at the last UDF/plugin uninstallation.

Updated the tests to mention the error name next to the error code.