SLING-3588 - Sling console: keep console history since start

The SlingConsoleEventListener now keeps a cache of at most 500 events in
memory if the Sling console is not open. The events are printed as soon
as the console is open.

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1598288 13f79535-47bb-0310-9956-ffa450edef68