Header key for scheduled message delivery should be defined

[gaohoward]

To set a scheduled message, one has to use the key:

message.setLongProperty(MessageImpl.HDR_SCHEDULED_DELIVERY_TIME.toString(), time);

The header HDR_SCHEDULED_DELIVERY_TIME is defined on MessageImpl, which should be an internal class. It would be better to define it on JBossMessage?

Also the header is defined as SimpleString, which is also an internal object. It should be a plain String object.

[timfox]

MessageImpl is as core class, JBossMessage is a JMS layer class.

We can't have core depend on JMS, however if you can think of a better place to put it fine.