Avoid a slow exception catch in JdkMapAdapterStringMap constructor

[jdgenerix]

The org.apache.logging.log4j.core.impl.JdkMapAdapterStringMap constructor is called from org.apache.logging.log4j.core.impl.ThreadContextDataInjector with an UnmodiableMap (copy) and the constructor is trying to modify and catch an exception to detect if this is an immutable Map.
This code is triggered a lot of time and could be more efficient.
Adding a package protected constructor which take the immutable boolean flag as an argument and using it from ThreadContextDataInjector would avoid this slow try catch.

        @Override
        public ReadOnlyStringMap rawContextData() {
            final ReadOnlyThreadContextMap map = ThreadContext.getThreadContextMap();
            if (map instanceof ReadOnlyStringMap) {
                return (ReadOnlyStringMap) map;
            }
            // note: default ThreadContextMap is null
            final Map<String, String> copy = ThreadContext.getImmutableContext();
            return copy.isEmpty() ? ContextDataFactory.emptyFrozenContextData() : new JdkMapAdapterStringMap(copy);
        }


        public JdkMapAdapterStringMap(final Map<String, String> map) {
		this.map = Objects.requireNonNull(map, "map");
		try {
			map.replace(Strings.EMPTY, Strings.EMPTY, Strings.EMPTY);
		} catch (final UnsupportedOperationException ignored) {
			immutable = true;
		}
	}