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/_client/updates.py

@@ -206,7 +206,7 @@ async def _update_loop(self: 'TelegramClient'):
         # Entities are not saved when they are inserted because this is a rather expensive
         # operation (default's sqlite3 takes ~0.1s to commit changes). Do it every minute
         # instead. No-op if there's nothing new.
-        await self.session.save()
+        await self._session.save()
 
         # We need to send some content-related request at least hourly
         # for Telegram to keep delivering updates, otherwise they will
@@ -231,33 +231,6 @@ async def _dispatch_queue_updates(self: 'TelegramClient'):
     self._dispatching_updates_queue.clear()
 
 async def _dispatch_update(self: 'TelegramClient', update, entities, others, channel_id, pts_date):
-    if entities:
-        rows = self._entity_cache.add(list(entities.values()))
-        if rows:
-            await self.session.insert_entities(rows)
-
-    if not self._entity_cache.ensure_cached(update):
-        # We could add a lock to not fetch the same pts twice if we are
-        # already fetching it. However this does not happen in practice,
-        # which makes sense, because different updates have different pts.
-        if self._state_cache.update(update, check_only=True):
-            # If the update doesn't have pts, fetching won't do anything.
-            # For example, UpdateUserStatus or UpdateChatUserTyping.
-            try:
-                await _get_difference(self, update, entities, channel_id, pts_date)
-            except OSError:
-                pass  # We were disconnected, that's okay
-            except RpcError:
-                # There's a high chance the request fails because we lack
-                # the channel. Because these "happen sporadically" (#1428)
-                # we should be okay (no flood waits) even if more occur.
-                pass
-            except ValueError:
-                # There is a chance that GetFullChannel and GetDifference
-                # inside the _get_difference() function will end up with
-                # ValueError("Request was unsuccessful N time(s)") for whatever reasons.
-                pass
-
     built = EventBuilderDict(self, update, entities, others)
 
     for builder, callback in self._event_builders: