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).

Conflicts:
	components/nexus-core/src/main/java/org/sonatype/nexus/threads/MDCUtils.java
	components/nexus-core/src/main/java/org/sonatype/nexus/web/internal/SecurityFilter.java
fix isSet negate entire expression
Add tests for UserIdMdcHelper
Add tests for UserIdMdcHelper
Add tests for UserIdMdcHelper
Use set() not setIfNeeded()
Merge pull request #308 from sonatype/backport-set-mdc-userid

REVIEW (2.7): Backport set mdc userid changes