A few cleanups to make init and tear-down clearer for both NonJavaThreads and JavaThreads. In particular:

- Move NJT-list management to init/tear-down of the active thread, instead of constructor/destructor
- Factor out common code across different kinds of NJTs
- Make it easier to add init/teardown code across thread types

Proposed approach in non-virtual call_run:

void call_run() {
  // common init for all threads
  ...
  // virtual init
  this->pre_run();

  this_run();

  // common teardown

  // virtual teardown
  this->post_run()
  
  // Note: can't reference 'this' here as post_run may have deleted it
  ...
}