Avoid caching event loop in telethon.sync

This would cause issues when creating and setting a different
event loop from a different thread since it would use the old
loop and not the new one, solved by explicitly getting the
loop every time, although this has a slight performance hit.

telethon/sync.py

@@ -25,8 +25,7 @@ from .tl.custom.sendergetter import SenderGetter
 
 
 class _SyncGen:
-    def __init__(self, loop, gen):
-        self.loop = loop
+    def __init__(self, gen):
         self.gen = gen
 
     def __iter__(self):
@@ -34,21 +33,25 @@ class _SyncGen:
 
     def __next__(self):
         try:
-            return self.loop.run_until_complete(self.gen.__anext__())
+            return asyncio.get_event_loop() \
+                .run_until_complete(self.gen.__anext__())
         except StopAsyncIteration:
             raise StopIteration from None
 
 
-def _syncify_wrap(t, method_name, syncifier):
+def _syncify_wrap(t, method_name, gen):
     method = getattr(t, method_name)
 
     @functools.wraps(method)
     def syncified(*args, **kwargs):
         coro = method(*args, **kwargs)
-        return (
-            coro if asyncio.get_event_loop().is_running()
-            else syncifier(coro)
-        )
+        loop = asyncio.get_event_loop()
+        if loop.is_running():
+            return coro
+        elif gen:
+            return _SyncGen(coro)
+        else:
+            return loop.run_until_complete(coro)
 
     # Save an accessible reference to the original method
     setattr(syncified, '__tl.sync', method)
@@ -61,14 +64,13 @@ def syncify(*types):
     into synchronous, which return either the coroutine or the result
     based on whether ``asyncio's`` event loop is running.
     """
-    loop = asyncio.get_event_loop()
     for t in types:
         for name in dir(t):
             if not name.startswith('_') or name == '__call__':
                 if inspect.iscoroutinefunction(getattr(t, name)):
-                    _syncify_wrap(t, name, loop.run_until_complete)
+                    _syncify_wrap(t, name, gen=False)
                 elif isasyncgenfunction(getattr(t, name)):
-                    _syncify_wrap(t, name, functools.partial(_SyncGen, loop))
+                    _syncify_wrap(t, name, gen=True)
 
 
 syncify(TelegramClient, Draft, Dialog, MessageButton,