Drop support for multithreaded PMs

This feature was only partially implemented (only some EC methods handle it, no SCO container methods handle it) and is of little value to any right-minded person. One option is to remove all EC locking and be done with it. 
Note that the JDO spec does not mark this as an "optional" feature, so maybe a better option is to further isolate any multithread support over what was done in 5.2->6.0