message: Improve reliability of sync_with_ping()
  Only allow 1 txAdmissionThread and 1 msgHandlerThread
diff: @@ -137,10 +137,10 @@ class AcceptBlockTest(BitcoinTestFramework):
  # from peers which are not whitelisted, while Node1 will be used for
  # the whitelisted case.
  self.nodes = []
  - self.nodes.append(start_node(0, self.options.tmpdir, ["-debug=net", "-debug=req"],
  + self.nodes.append(start_node(0, self.options.tmpdir, ["-debug=net", "-debug=req", "-net.txAdmissionThreads=1", "-net.msgHandlerThreads=1"],
  binary=self.options.testbinary))
  self.nodes.append(start_node(1, self.options.tmpdir,
  - ["-debug=net", "-debug=req", "-whitelist=127.0.0.1"],
  + ["-debug=net", "-debug=req", "-whitelist=127.0.0.1", "-net.txAdmissionThreads=1", "-net.msgHandlerThreads=1"],
  binary=self.options.testbinary))
  def run_test(self):
  @@ -243,7 +243,6 @@ class AcceptBlockTest(BitcoinTestFramework):
  headers_message.headers.append(CBlockHeader(next_block))
  tips[j] = next_block
  test_node.sync_with_ping()
  - time.sleep(2)
  for x in all_blocks:
  try:
  @@ -260,7 +259,7 @@ class AcceptBlockTest(BitcoinTestFramework):
  white_node.send_message(headers_message) # Send headers leading to tip
  white_node.send_message(msg_block(tips[1])) # Now deliver the tip
  try:
  - time.sleep(2) # give time for the tip to be delivered
  + white_node.sync_with_ping()
  self.nodes[1].getblock(tips[1].hash)
  print("Unrequested block far ahead of tip accepted from whitelisted peer")
  except:
  