[NEXUS-5662] Add servlet filter to add userId to MDC
Merge branch 'master' into NEXUS-5662-userid-mdc

Conflicts:
	plugins/siesta/nexus-siesta-plugin/src/main/java/org/sonatype/nexus/plugins/siesta/SiestaModule.java
Merge pull request #816 from sonatype/NEXUS-5662-userid-mdc

REVIEW: Add servlet filter to add userId to MDC