Completely overhaul connections and transports

Reduce abstraction leaks. Now the transport can hold any state,
rather than just the tag. It's also responsible to initialize on
the first connection, and they can be cleanly reset.

asyncio connections are no longer used, in favour of raw sockets,
which should avoid some annoyances.

For the time being, more obscure transport modes have been removed,
as well as proxy support, until further cleaning is done.

telethon/_network/transports/intermediate.py

@@ -0,0 +1,29 @@
+from .transport import Transport
+import struct
+
+
+class Intermediate(Transport):
+    def __init__(self):
+        self._init = False
+
+    def recreate_fresh(self):
+        return type(self)()
+
+    def pack(self, input: bytes) -> bytes:
+        if self._init:
+            header = b''
+        else:
+            header = b'\xee\xee\xee\xee'
+            self._init = True
+
+        return header + struct.pack('<i', len(data)) + data
+
+    def unpack(self, input: bytes) -> (int, bytes):
+        if len(input) < 4:
+            raise EOFError()
+
+        length = struct.unpack('<i', input[:4])[0] + 4
+        if len(input) < length:
+            raise EOFError()
+
+        return length, input[4:length]