Drop support for multithreaded PMs

[andyjefferson]

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