Fix backup plugin

Update required Python version
Update pyln and other packages
Update README

backup/protocol.py

@@ -0,0 +1,77 @@
+'''
+Socket-based remote backup protocol. This is used to create a connection to a backup backend, and send it incremental database updates.
+'''
+import socket
+import struct
+from typing import Tuple
+import zlib
+
+from backend import Change
+
+
+class PacketType:
+    CHANGE = 0x01
+    SNAPSHOT = 0x02
+    REWIND = 0x03
+    REQ_METADATA = 0x04
+    RESTORE = 0x05
+    ACK = 0x06
+    NACK = 0x07
+    METADATA = 0x08
+    DONE = 0x09
+    COMPACT = 0x0a
+    COMPACT_RES = 0x0b
+
+
+PKT_CHANGE_TYPES = {PacketType.CHANGE, PacketType.SNAPSHOT}
+
+
+def recvall(sock: socket.socket, n: int) -> bytearray:
+    '''Receive exactly n bytes from a socket.'''
+    buf = bytearray(n)
+    view = memoryview(buf)
+    ptr = 0
+    while ptr < n:
+        count = sock.recv_into(view[ptr:])
+        if count == 0:
+            raise IOError('Premature end of stream')
+        ptr += count
+    return buf
+
+
+def send_packet(sock: socket.socket, typ: int, payload: bytes) -> None:
+    sock.sendall(struct.pack('!BI', typ, len(payload)))
+    sock.sendall(payload)
+
+
+def recv_packet(sock: socket.socket) -> Tuple[int, bytes]:
+    (typ, length) = struct.unpack('!BI', recvall(sock, 5))
+    payload = recvall(sock, length)
+    return (typ, payload)
+
+
+def change_from_packet(typ, payload):
+    '''Convert a network packet to a Change object.'''
+    if typ == PacketType.CHANGE:
+        (version, ) = struct.unpack('!I', payload[0:4])
+        payload = zlib.decompress(payload[4:])
+        return Change(version=version, snapshot=None,
+                      transaction=[t.decode('UTF-8') for t in payload.split(b'\x00')])
+    elif typ == PacketType.SNAPSHOT:
+        (version, ) = struct.unpack('!I', payload[0:4])
+        payload = zlib.decompress(payload[4:])
+        return Change(version=version, snapshot=payload, transaction=None)
+    raise ValueError('Not a change (typ {})'.format(typ))
+
+
+def packet_from_change(entry):
+    '''Convert a Change object to a network packet.'''
+    if entry.snapshot is None:
+        typ = PacketType.CHANGE
+        payload = b'\x00'.join([t.encode('UTF-8') for t in entry.transaction])
+    else:
+        typ = PacketType.SNAPSHOT
+        payload = entry.snapshot
+
+    version = struct.pack("!I", entry.version)
+    return typ, version + zlib.compress(payload)