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': [],

test/lsp/test_update_config.vader

@@ -0,0 +1,17 @@
+Before:
+  runtime autoload/ale/lsp.vim
+
+  let g:conn_id = ale#lsp#Register('executable', '/foo/bar', {})
+
+After:
+  Restore
+
+  unlet! g:conn_id
+
+  runtime autoload/ale/lsp.vim
+
+Execute(Only send updates when the configuration dictionary changes):
+  AssertEqual 0, ale#lsp#UpdateConfig(g:conn_id, bufnr(''), {})
+  AssertEqual 1, ale#lsp#UpdateConfig(g:conn_id, bufnr(''), {'a': 1})
+  AssertEqual 0, ale#lsp#UpdateConfig(g:conn_id, bufnr(''), {'a': 1})
+  AssertEqual 1, ale#lsp#UpdateConfig(g:conn_id, bufnr(''), {})