Switch Content Hashing from CRC32 to XXHash64

[ahirreddy]

• CRC32 collision probability is too high for comfort since it's only 32-bits
• Switching to XXHash64 since its a high quality hash, 64-bits and extremely fast (faster than CRC32
  for medium size inputs: https://lz4.github.io/lz4-java/1.3.0/xxhash-benchmark/)