Add a lock util

Realized that messages could be sent over the same stream while we're
waiting for a response for the previous message. This is not good. Made
a lock util to help with that.

I see this happening a lot once we start trying to do completion type
items, so that's where I came up with this part.

autoload/langserver/lock.vim

@@ -0,0 +1,16 @@
+
+function! s:dict_lock() dict
+  let self.locked = v:true
+endfunction
+
+function! s:dict_unlock() dict
+  let self.locked = v:false
+endfunction
+
+function! langserver#lock#semaphore() abort
+  let l:ret = {}
+  let l:ret.locked = v:false
+  let l:ret.lock = function('s:dict_lock')
+  let l:ret.unlock = function('s:dict_unlock')
+  return l:ret
+endfunction

tests/test_lock.vader

@@ -0,0 +1,10 @@
+Execute (Test Lock):
+  let my_lock = langserver#lock#semaphore()
+
+  AssertEqual v:false, my_lock.locked
+
+  call my_lock.lock()
+  AssertEqual v:true, my_lock.locked
+
+  call my_lock.unlock()
+  AssertEqual v:false, my_lock.locked