Tests inside wcfs/ care to do this, but e.g. test.py/fs-wcfs autospawns
wcfs servers during regular bigfile tests. If we don't stop spawned
wcfs, those processes will leak, and also they keep `nxdtest
test.py/*-wcfs` in "hung" state, because nxdtest is waiting for wcfs to
stop as wcfs stdout is connected to nxdtest input.

Currently kills wcfs in abrupt way, because graceful pinner shutdown is
not yet implemented there.