Silence exceptions from qpid connection.close()

Fixes bug 1185465

Don't log exceptions that happen during connection.close().  They're
not surprising (a closed socket can't be read or written), don't
require sysadmin attention, and provide red herrings in the logs
to distract from finding real root causes of problems.

Change-Id: Ie414ba3f4ff1f1dee13e6168ac2e28dab535581b