gopls: prevent multiple restart language server requests

[hyangah]

From golang/go#43023.
The trace shows that multiple outstanding language server restart requests confuses subsequent language client start process.

[Trace - 20:53:24.387 PM] Sending request 'shutdown - (35)'.
Params: 

...
[Trace - 21:07:06.546 PM] Received response 'shutdown - (35)' in 822159ms.
Result: null

While the first shutdown request is blocked waiting for the gopls's response, extra restart requests were made.
As a result, as soon as the language server was restarted eventually, unblocked languageClient.stop calls are followed by dispose that race with subsequent start and mess with the restart sequence.
When the connection is set up dispose triggers shutdown requests. See multiple shutdown requests were sent to the language server back to back. (message id 1 and 2) immediately following the initialize request.

[Trace - 21:07:06.546 PM] Received notification 'window/logMessage'.
Params: {"type":3,"message":"2020/12/05 21:07:06 Shutdown session\n\tshutdown_session=1\n"}

[Error - 下午9:07:06] Starting client failed
Error: Connection is already listening
	at throwIfListening (c:\Users\xiaoqing\.vscode\extensions\golang.go-0.19.0\dist\goMain.js:94014:19)
	at Object.listen (c:\Users\xiaoqing\.vscode\extensions\golang.go-0.19.0\dist\goMain.js:94344:13)
	at Object.listen (c:\Users\xiaoqing\.vscode\extensions\golang.go-0.19.0\dist\goMain.js:10479:34)
	at c:\Users\xiaoqing\.vscode\extensions\golang.go-0.19.0\dist\goMain.js:12535:24
[Error - 下午9:07:06] Starting client failed
Error: Connection is already listening
	at throwIfListening (c:\Users\xiaoqing\.vscode\extensions\golang.go-0.19.0\dist\goMain.js:94014:19)
	at Object.listen (c:\Users\xiaoqing\.vscode\extensions\golang.go-0.19.0\dist\goMain.js:94344:13)
	at Object.listen (c:\Users\xiaoqing\.vscode\extensions\golang.go-0.19.0\dist\goMain.js:10479:34)
	at c:\Users\xiaoqing\.vscode\extensions\golang.go-0.19.0\dist\goMain.js:12535:24
[Trace - 21:07:06.706 PM] Sending request 'initialize - (0)'.
Params: {"processId":29660,"clientInfo": ... }

[Trace - 21:07:06.717 PM] Sending request 'shutdown - (1)'.
Params: 


[Trace - 21:07:06.717 PM] Sending request 'shutdown - (2)'.
Params: 


[Trace - 21:07:06.719 PM] Received response 'shutdown - (1)' in 1ms.
Result: null


[Trace - 21:07:06.719 PM] Received notification 'window/logMessage'.
Params: {"type":3,"message":"2020/12/05 21:07:06 server shutdown without initialization\n"}

[Trace - 21:07:06.719 PM] Received response 'shutdown - (2)' in 1ms.
Result: null

We need to prevent additional restart language server request while the previous language server restart request is still pending.

[gopherbot]

Change https://golang.org/cl/276973 mentions this issue: src/goLanguageServer: show language server start progress