Several unit-tests-related fixes & improvements.

1. Changed hived_fixture so that uncaught exceptions from unit test are handled nominally by BOOST. Only the exceptions during appbase finish stop unit tests now as they should be fixed before running another test.
2. application "singleton" has been improved to make sure that the instance is restarted once its finish() method was called.
3. Several issues reported by valgrind were fixed:
   • Custom deleter has been added to chain_plugin_impl to allow correct deletion of the object of the class invisible in header file where it's stored in unique_ptr.
   • Skip flags final check has been moved to occcur before appbase finish.
   • Node properties related API in database class has been reduced to get/set_node_skip_flags to avoid sharing the object reference around. This would save much time investigating another valgrind report (if applied earlier) as the report would show exactly where the problem was.