Fix for Bug 943

Fix that snmpj didn't provide method to close socket, and cause snmp listening port can't be free.

(cherry picked from commit 8e405b20a3cfcbffc498d1448ddc73133649b0de)

Change-Id: I10d648d302f93561b1ca1e145859759f1b0caf19
Signed-off-by: ylhsieh <ylhsieh@itri.org.tw>