text: Title: [ChunkStore] Decompression error on concurrent read & write
  Body: #### Arctic Version
  
  ```
  1.70.0
  ```
  
  #### Arctic Store
  
  ```
  ChunkStore
  ```
  
  #### Platform and version
  
  CentOS 7.2
  
  #### Description of problem and/or code sample that reproduces the issue
  I'm getting an error when I have two threads simultaneously reading and writing from one symbol. The reading thread will periodically throw a LZ4BlockError:
  
  ```LZ4BlockError: Decompression failed: corrupt input or insufficient space in destination buffer. Error code: 24049```
  
  Here is the code to reproduce it:
  ```
  def get_library():
      return Arctic('some_host:1234').get_library(LIB_NAME)
  
  def write_loop():
      library = get_library()
      while True:
          data = create_data(100)  # Creates 100 rows of random data
          library.append('SYMBOL', data)
          
  def read_loop():
      library = get_library()
      while True:
          df = library.read('SYMBOL')
  
  proc = mp.Process(target=write_loop)
  proc.start()
  
  try:
      read_loop()
  finally:
      proc.terminate()
  ```
  
  From a quick check, it seems that the data being passed to decompress(_str) in _compression.py is not valid lz4 - could the the block metadata and data be out of sync?