Add protection against calling invalid session in DelegatingHttpSession

[pzoio]

Have seen cases of the below:

Caused by: java.lang.IllegalStateException: Invalid for read: id=node01987v04vzgtt31dnkyxtt1gefp1 created=1596809111265 accessed=1596810649655 lastaccessed=1596810647928 maxInactiveMs=1800000 expiry=1596812449655
    at org.eclipse.jetty.server.session.Session.checkValidForRead(Session.java:692)
    at org.eclipse.jetty.server.session.Session.getAttribute(Session.java:715)
    at org.impalaframework.web.servlet.wrapper.session.DelegatingHttpSession.getAttribute(DelegatingHttpSession.java:41)
    at org.impalaframework.web.servlet.wrapper.session.PartitionedHttpSession.getAttribute(PartitionedHttpSession.java:61)
    at org.impalaframework.web.servlet.wrapper.session.StateProtectingHttpSession.getAttribute(StateProtectingHttpSession.java:74)

Not sure what the root cause is, but this adds some protection.