QPID-6152: [Java Client] Allow connection/session close time to be overridden by system property.

Change implementation so that the timeout is applied individually to each session close, the connection and
the shutdown of the task pool.   Also moved the shutdown of the task pool to a finally, so that it occurs
even if one or more sessions fail to close.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1631810 13f79535-47bb-0310-9956-ffa450edef68