A finer-grained import lock

Hello,

This patch fixes the long-standing issue of deadlocks with a
combination of starting threads and importing modules. It also makes
PyImport_ImportModuleNoBlock() basically useless. The idea is to have a
separate lock for each module being imported, and only use the global
import lock around a couple of operations (such as creating the module
locks themselves).

http://bugs.python.org/issue9260

Regards

Antoine.