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/connection.py

@@ -0,0 +1,61 @@
+import asyncio
+import socket
+
+from .transports.transport import Transport
+
+
+CHUNK_SIZE = 32 * 1024
+
+
+# TODO ideally the mtproto impl would also be sans-io, but that's less pressing
+class Connection:
+    def __init__(self, ip, port, *, transport: Transport, loggers, local_addr=None):
+        self._ip = ip
+        self._port = port
+        self._log = loggers[__name__]
+        self._local_addr = local_addr
+
+        self._sock = None
+        self._in_buffer = bytearray()
+        self._transport = transport
+
+    async def connect(self, timeout=None, ssl=None):
+        """
+        Establishes a connection with the server.
+        """
+        loop = asyncio.get_event_loop()
+
+        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+        sock.setblocking(False)
+        if self._local_addr:
+            sock.bind(self._local_addr)
+
+        await asyncio.wait_for(loop.sock_connect(sock, (self._ip, self._port)), timeout)
+        self._sock = sock
+
+    async def disconnect(self):
+        self._sock.close()
+        self._sock = None
+
+    async def send(self, data):
+        if not self._sock:
+            raise ConnectionError('not connected')
+
+        loop = asyncio.get_event_loop()
+        await loop.sock_sendall(self._sock, self._transport.pack(data))
+
+    async def recv(self):
+        if not self._sock:
+            raise ConnectionError('not connected')
+
+        loop = asyncio.get_event_loop()
+        while True:
+            try:
+                length, body = self._transport.unpack(self._in_buffer)
+                del self._in_buffer[:length]
+                return body
+            except EOFError:
+                self._in_buffer += await loop.sock_recv(self._sock, CHUNK_SIZE)
+
+    def __str__(self):
+        return f'{self._ip}:{self._port}/{self._transport.__class__.__name__}'