724d2f7 Limit LogEscape string size (Russell Yanofsky)

Pull request description:

  Try to avoid massive debug log file sizes, and travis MemoryError in
  bitcoin/bitcoin#10102, https://travis-ci.org/bitcoin/bitcoin/jobs/558040257:

  ```
  TestFramework (ERROR): Unexpected exception caught during testing
  Traceback (most recent call last):
    File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/test_framework/test_framework.py", line 193, in main
      self.run_test()
    File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/feature_block.py", line 1222, in run_test
      self.send_blocks(blocks, True, timeout=1500)
    File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/feature_block.py", line 1373, in send_blocks
      self.nodes[0].p2p.send_blocks_and_test(blocks, self.nodes[0], success=success, reject_reason=reject_reason, force_send=force_send, timeout=timeout, expect_disconnect=reconnect)
    File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/test_framework/mininode.py", line 553, in send_blocks_and_test
      assert node.getbestblockhash() != blocks[-1].hash
    File "/usr/lib/python3.6/contextlib.py", line 88, in __exit__
      next(self.gen)
    File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/test_framework/test_node.py", line 318, in assert_debug_log
      log = dl.read()
    File "/usr/lib/python3.6/codecs.py", line 321, in decode
      (result, consumed) = self._buffer_decode(data, self.errors, final)
  MemoryError
  ```

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Tree-SHA512: 4dcc98ad0db7d32c23de67ac2166ba33d15be5f1824363e3642015141489f9b11f5d55fe454ddea78a38be86bfea12aa1d7e490754db4d7ede63e9992d4da315