Only send LSP config updates when the dict changes

Each LSP connection now stores its configuration dictionary. It is
initially empty (`{}`) and is updated each time the LSP connection is
started. When a change is detected, the workspace/didChangeConfiguration
message is sent to the LSP servers with the updated configuration.

test/lsp/test_other_initialize_message_handling.vader

@@ -7,6 +7,7 @@ Before:
   \ 'initialized': 0,
   \ 'init_request_id': 0,
   \ 'init_options': {},
+  \ 'config': {},
   \ 'callback_list': [],
   \ 'message_queue': [],
   \ 'capabilities_queue': [],