[REVIEW] NEXUS-5728: Fixing ehcache.xml configuration

The cache config used by Shiro EIS DAO, named shiro-activeSessionCache was badly configured that led to "lost sessions".

Verified (see issue comments) that after this change to ehcache.xml, reproducing the stack trace with `org.apache.shiro.session.UnknownSessionException` was no longer possible.

Issue:
https://issues.sonatype.org/browse/NEXUS-5728