Fix infinite loop when SocketServerCollector receives invalid packet

[mroch]

When in --ip-mode=server, if a rogue client connects, it causes the server to go into an infinite loop trying to "retry" reading in _blockingread: https://github.com/matthewwall/mtools/blob/fa488f98b48bb734c2be94ef4ad4f24c56790443/bin/btmon.py#L2324-L2347

READ_RETRIES == 0. for each invalid character, _read raises ReadError, so it keeps reading until it hits EOF. then it raises an EmptyReadError and waits RETRY_WAIT (60 seconds), then tries to _read from the still-closed socket. This errors, ad nauseum.

This behavior makes sense for serial, since the only way to synchronize is to consume until you hit the packet header. But IP packets will never recover, so we don't need the _blockingread behavior and can just call _read.

There was one further issue: if the client disconnects quickly, then self._conn.shutdown() throws because the socket is already closed. shutdown is unnecessary in this case so we just ignore exceptions.

Test:

• run bin/btmon.py --packet-format=gem48ptbin --ip --ip-mode=server --ip-port=5000 --debug --print

• run telnet localhost 5000, send something like HELLO, ^], ctrl-d to disconnect

  2020/10/20 17:41:42 SOCKET: waiting for connection
  2020/10/20 17:41:45 reading 1 of 1 packets
  2020/10/20 17:41:51 SOCKET: read 1 of 1 bytes from socket: 48
  2020/10/20 17:41:51 expected START_HEADER0 0xfe, got 0x48
  Traceback (most recent call last):
    File "bin/btmon.py", line 2568, in read
      self._read(packet_format)
    File "bin/btmon.py", line 2318, in _read
      packets.extend(packet_format.read(self))
    File "bin/btmon.py", line 1675, in read
      return self._read1(collector, self.DATA_BYTES_LENGTH, self.PACKET_ID)
    File "bin/btmon.py", line 1563, in _read1
      self._checkbyte(data, 'START_HEADER0', self.START_HEADER0)
    File "bin/btmon.py", line 1558, in _checkbyte
      (label, hex(evalue), hex(b)))
  ReadError: expected START_HEADER0 0xfe, got 0x48
  2020/10/20 17:41:51 SOCKET: closing connection
  2020/10/20 17:41:51 buffer info:
  2020/10/20 17:41:51 processing with PrintProcessor
  2020/10/20 17:41:51 SOCKET: waiting for connection
  

• run telnet again, immediately ^], ctrl-d without sending anything

  2020/10/20 17:43:31 reading 1 of 1 packets
  2020/10/20 17:43:34 SOCKET: read 0 of 1 bytes from socket:
  2020/10/20 17:43:34 SOCKET: client closed connection unexpectedly
  2020/10/20 17:43:34 SOCKET: closing connection
  2020/10/20 17:43:34 buffer info:
  2020/10/20 17:43:34 processing with PrintProcessor
  2020/10/20 17:43:34 SOCKET: waiting for connection
  

Fixes #22