Simple implementation of LSP protocol shutdown request and exit notification handlers

[shivam71]

Changes related to Netbeans Java Language Server

Context

• LSP Protocol provides the language clients to send messages to the language server to shutdown and exit .
• This allows language clients to control the lifecycle of the language server completely from start to shutdown. Spec relevant section : lifeCycleMessages

About the changes made

• Shutdown Request handler and the Exit Notification handler implemented

Note
As per the LSP Spec about Shutdown Request the language server should respond with Invalid Request error post receiving the shutdown request . That can be done in the future if required but has been deferred for now to keep this change simple .

[lahodaj]

Looks reasonable to me. I was thinking if using something like AtomicBoolean would be safer, but I think the dispatch is done in one thread only, so plain boolean is probably OK.

[matthiasblaesing]

@lahodaj @shivam71 is not a committer, if you still think that this is good to go, please merge.