Give thisAccessedTime and lastAccessedTime for sessions

Give thisAccessedTime and lastAccessedTime for sessions
a clear semantics:

- thisAccessedTime will be updated at the beginning and
  at the end of session use
- lastAccessedTime will only be updated at the end of
  session use

This means:

- lastAccessedTime is the last access time of a session
  disregarding any request still being processed on.
  So this is good to use even from within a request
  to detect when its own session has been used last before.
- thisAccessedTime already gets updated when a new request
  disregarding any request still being processed on.
  So this is better for any idleness check or information.
- thisAccessedTime >= lastAccessedTime always

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@711711 13f79535-47bb-0310-9956-ffa450edef68