RFR 8005704: Update ConcurrentHashMap to v8

[Martin Buchholz]

A long-returning size method seems like a framework-level decision.  We
could add a default method to Collection.java and Map.java

default long mappingCount() { return size(); }

I'm not sure mappingCount is the best name, but all names will be
confusing.  Using the name "length()" at least has an onomatopoeic mnemonic
advantage - it suggests that it might return a long.

    /**
     * Returns the number of mappings. This method should be used
     * instead of {@link #size} because a ConcurrentHashMap may
     * contain more mappings than can be represented as an int. The
     * value returned is an estimate; the actual count may differ if
     * there are concurrent insertions or removals.
     *
     * @return the number of mappings
     */
    public long mappingCount() {
        long n = sumCount();
        return (n < 0L) ? 0L : n; // ignore transient negative values
    }