Without special care a forked child may interfere in parent-wcfs
exchange via Python GC -> PyFileH.__del__ -> FileH.close -> message to
WCFS sent from the child. This actually happens for real when running
test.py/neo-wcfs because NEO test cluster spawns master and storage
nodes with just fork without exec.

-> detach from wcfs in child right after fork and deactivate all
mappings in order not to provide stale data. See top-level comments
added to wcfs/client/wcfs.cpp for details.