public void purgeStaleEntries() {
        for (int i = 0; i < segments.length; ++i)
            segments[i].removeStale();
    }
Removes any stale entries whose keys have been finalized. Use of this
method is normally not necessary since stale entries are automatically
removed lazily, when blocking operations are required. However, there
are some cases where this operation should be performed eagerly, such
as cleaning up old references to a ClassLoader in a multi-classloader
environment.

Note: this method will acquire locks, one at a time, across all segments
of this table, so if it is to be used, it should be used sparingly.
commons-profiler/src/main/java/com/navercorp/pinpoint/common/profiler/concurrent/jsr166/ConcurrentReferenceHashMap.java