[SNAP-3270] removing streaming query listener in finalize method

[vatsalmevada]

What changes were proposed in this pull request?

For streaming UI, SnappyStreamingQueryListener is registered on listener bus
at the time of creating SnappySession. However, this listener is never removed
from the listener bus. Hence even if the SnappySession is collected by GC,
SnappyStreamingQueryListener is left orphan on the listener bus which is never
eligible for GC collection.

To fix this we are removing the listener from the listener bus when the session
the instance is getting collected by GC (i.e. in finalize method) which will make
the listener instance eligible for GC during the next GC cycle.

It should be OK if the listener instance gets collected in the next GC cycle as the
the memory footprint of the listener object is not big.

Another possible place to remove the listener in close method of the session,
however close method of session is not required to be closed explicitly.

How was this patch tested?

Reproduced the issue by running the following code as part of a snappy job:

while(true){
      session.newSession()
}

Collected histogram of leader process using jmap and observed that instances of
SnappyStreamingQueryListener is increasing indefinitely and never garbage
collected whereas SnappySession instances are garbage collected:

jmap -histo:live <leader pid>|grep "SnappySession\|SnappyStreamingQueryListener"

Followed the same steps after applying the changes and noticed that
SnappyStreamingQueryListener instances are garbage collected.

---

• precheckin -Pspark
• SnappyStreamingQueryListenerSuite is passing now. Added a call to finalize
  method in StreamingQueryListenerSuite to get this change tested.

[dshirish]

LGTM