Set userId in MDC on login, current filter impl of MDC handling is not sufficent
Add custom WebSecurityManager impl and override login/logout to set MDC userId attribute.
Drop setting of MDC userId attribute from authc filter.
Split off MDC userId attribute from MDCUtils (nexus-core) into UserIdMdcHelper (nexus-security).
fix isSet negate entire expression
Add tests for UserIdMdcHelper
Add tests for UserIdMdcHelper
Add tests for UserIdMdcHelper
Merge pull request #302 from sonatype/set-mdc-userid-onlogin

REVIEW: Set userId in MDC on login