fix(messages)!: vim.ui_attach message callbacks are unsafe

Problem:  Lua callbacks for "msg_show" events with vim.ui_attach() are
          executed when it is not safe.
Solution: Disallow non-fast API calls for "msg_show" event callbacks.
          Automatically detach callback after excessive errors.
          Make sure fast APIs do not modify Nvim state.

test/functional/lua/loop_spec.lua

@@ -87,9 +87,9 @@ describe('vim.uv', function()
     screen:expect([[
                                                         |
       {2:                                                  }|
-      {3:Error executing luv callback:}                     |
+      {3:Error executing callback:}                         |
       {3:[string "<nvim>"]:5: E5560: nvim_set_var must not }|
-      {3:be called in a lua loop callback}                  |
+      {3:be called in a fast event context}                 |
       {3:stack traceback:}                                  |
       {3:        [C]: in function 'nvim_set_var'}           |
       {3:        [string "<nvim>"]:5: in function <[string }|