Fix deadlock in child threads launched from python
Fix deadlock in child threads launched from python

Pybind11 seems to have an issue in gil_scoped_acquire when being
called from child threads created from python. This resulted in a deadlock here:
https://github.com/searchivarius/nmslib/issues/291

A simple workaround is to use the PyGILState_Ensure from the python c-api directly
instead.

nmslib/nmslib,nmslib/nmslib,nmslib/nmslib,nmslib/nmslib,nmslib/nmslib,nmslib/nmslib,nmslib/nmslib