logback MDC userid should not be cloned permanently in spawned threads

Certain admin actions spawn scheduled task threads. Once thread gets "admin" userid in cloned MDC, it will be stuck with it forever as according to logback doco, the MDC is cloned in moment of creation, and stays detached. That's how it's "inherited". So, not a biggie, but still might be confusing while reading logs...

Expected, when thread is done its work and gets put back in pool, MDC is cleared.

related:
http://jira.qos.ch/browse/LOGBACK-624

http://jira.qos.ch/browse/LOGBACK-422

See example in https://issues.sonatype.org/browse/NEXUS-5749?focusedCommentId=193385&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-193385