Api fix & cdb2-open as debug msg-trap for test

[markhannum]

Fixes bug where cdb2api incorrectly donates a comdb2db fd resolved from comdb2db.cfg (which lists host information, but not tier information) to sockpool with an incorrect typestr. The test for this cdb2_open, brought in by the 'cdb2_open_comdb2db_plugins' plugins branch.