Previously, we had our own CRC that was almost but not quite like
zlib's implementation. However,
* it hasn't been subjected to the same rigor with regard to error-detection
properties and
* it may not even get used, depending upon whether zlib happens to get
loaded before or after liberasurecode.
Now, we'll use zlib's CRC-32 when writing new frags, while still
tolerating frags that were created with the old implementation.
Change-Id: Ib5ea2a830c7c23d66bf2ca404a3eb84ad00c5bc5
Closes-Bug: 1666320