TUI: do not use "nvim_get_option" in tui thread

Since "nvim_get_option" is executed on the tui thread as a C function
instead of msgpack-rpc, it accesses global variables that may change on the
main thread.

test/functional/ui/options_spec.lua

@@ -20,6 +20,8 @@ describe('UI receives option updates', function()
       pumblend=0,
       showtabline=1,
       termguicolors=false,
+      ttimeout=true,
+      ttimeoutlen=50,
       ext_cmdline=false,
       ext_popupmenu=false,
       ext_tabline=false,
@@ -108,6 +110,18 @@ describe('UI receives option updates', function()
       eq(expected, screen.options)
     end)
 
+    command("set nottimeout")
+    expected.ttimeout = false
+    screen:expect(function()
+      eq(expected, screen.options)
+    end)
+
+    command("set ttimeoutlen=100")
+    expected.ttimeoutlen = 100
+    screen:expect(function()
+      eq(expected, screen.options)
+    end)
+
     command("set all&")
     screen:expect(function()
       eq(defaults, screen.options)