Remove double-checked locking in ReadOnlyTrieMap

ReadOnlyTrieMap uses the double-checked locking (DCL) technique in this method:

protected Map<K, V> delegate() {
if (readOnly == null) {
synchronized (this) {
if (readOnly == null)

{ readOnly = readWrite.readOnlySnapshot(); }

}
}

return readOnly;
}

However DCL is not thread-safe. See http://www.javaworld.com/article/2074979/java-concurrency/double-checked-locking--clever--but-broken.html

A few solutions:

1) The simplest solution is to avoid lazy initialization and created readOnly in
the ctor and make it final.

2) If we really want lazy initialization the remove the DCL:

protected Map<K, V> delegate() {
synchronized (this) {
if (readOnly == null)

{ readOnly = readWrite.readOnlySnapshot(); }

}

return readOnly;
}

The downside to synchronizing on 'this' is that users may also synchronize
on the instance externally resulting in unwanted contention.

3) Keep the lazy initialization and DCL but make readOnly volatile. However this
would only be safe if readOnly is immutable (all fields final) or has all its
fields volatile.

DCL is usually not worth the trouble. Depending on usage, ie we can assume users will call delegate most of the time, I would opt for #1. Otherwise #2.