@Override
    public int referenceCount() {
        synchronized (contextMap) { return contextMap.size(); }
    }
Does reference counting by putting a unique key together with optional context in map
Used if system property jdisc.debug.resources=simple/true

@author baldersheim
jdisc_core/src/main/java/com/yahoo/jdisc/refcount/DebugReferencesByContextMap.java