CBG-4060: Role-based audit filtering

[bbrks]

CBG-4060

• Add GetRoles() to auth.User interface for access to roles (and initialization if nessesary)
• Add role filtering logic to shouldLogAuditEventForUserAndRole
• Store SG or CB Server roles on LogContext, but only when we have role-based filtering enabled
  • Getting the roles for CB Server RBAC users costs one additional HTTP /whoami request per-authed request when we have auditing and CB Server role filtering enabled.
  • Getting the roles for SG users is free if they are initialized
• Test coverage of role-filtered audit events in TestAuditLoggingFields

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2600/

[torcolvin]

LGTM