Enhancement to ThreadMessageContext enabling external tooling to retrieve the previous message context in a given invocation chain.

PREVIOUS_CONTEXT has been added to the ThreadMessageContext in core-spi. At any given time in a thread, PREVIOUS_CONTEXT represents it's immediate previous (old) message context in the thread. This is added strictly for the use of monitoring or other tooling, enabling the retrieval of the correct component name in case getService() returns a proxy. PREVIOUS_CONTEXT doesn't have any other meaning or use in the invocation chain other than this external tooling support.