Clean-up client's __init__ and remove entity cache

Entity cache uses are removed. It was a source of ever-growing memory
usage that has to be reworked. This affects everything that tried to
obtain an input entity, input sender or input chat (such as the
SenderGetter or calls to _get_entity_pair). Input entities need to be
reworked in any case.

Its removal also affects the automatic cache of any raw API request.

Raise last error parameter is removed, and its behaviour made default.

The connection type parameter has been removed, since users really have
no need to change it.

A few more attributes have been made private, since users should not
mess with those.

telethon/_events/common.py

@@ -147,8 +147,7 @@ class EventCommon(ChatGetter, abc.ABC):
         # TODO Nuke
         self._client = client
         if self._chat_peer:
-            self._chat, self._input_chat = utils._get_entity_pair(
-                self.chat_id, self._entities, client._entity_cache)
+            self._chat, self._input_chat = utils._get_entity_pair(self.chat_id, self._entities)
         else:
             self._chat = self._input_chat = None