SERVER-8388 scoped_db_conn_test threading issues

Made sure to wait for all listening threads to terminate before finishing one test case.
when choosing a user to put in system.profile, make best effort to use one from right database

Conflicts:
	src/mongo/db/introspect.cpp
Use PrincipalName::getDB() to find database name in introspect.cpp
fix test for new auth profiling name