https://issues.apache.org/jira/browse/AMQ-5015

Refactor the way sessions are pooled.  We don't need to keep the
PooledSession instances around since the state is unique to the session
it wraps we only need to keep the Session instances in the SessionPool
and create a new PooledSession on borrow to manage that session.  This
allows the PooledSession to have a real closed state that protects
against multiple close calls placing duplicate PooledSession instances
into the SessionPool.  This also simplifies the code in the
XaConnectionPool since it doesn't need to try and reset state in
PouledSessions before placing them back as it gets a fresh wrapper each
time with the correct state.