Thread-safe ValueBuffer

[xerial]

We need thread-safe ValueHolder so that we can safely pass the read value into another function. ValueBuffer is an example of its implementation. https://github.com/msgpack/msgpack-java/blob/v07-value-sf/msgpack-value/src/main/java/org/msgpack/value/ValueBuffer.java

[xerial]

Or Value interface should represent immutable (thread-safe) value, that is, materialized value. For traversing array/map efficiently, we can use MessageUnpacker.unapckArrayHeader and MessageUnpacker.unpackValue(...)