Previously, we were using a simple CRC32 for the IDs of rollup documents.
This is a very poor choice however, since 32bit IDs leads to collisions
between documents very quickly.
This commit moves Rollups over to a 128bit ID. The ID is a concatenation
of all the keys in the document (similar to the rolling CRC before),
hashed with 128bit Murmur3, then base64 encoded. Finally, the job
ID and a delimiter (`$`) are prepended to the ID.
This gurantees that there are 128bits per-job. 128bits should
essentially remove all chances of collisions, and the prepended
job ID means that _if_ there is a collision, it stays "within"
the job.
BWC notes:
We can only upgrade the ID scheme after we know there has been a good
checkpoint during indexing. We don't rely on a STARTED/STOPPED
status since we can't guarantee that resulted from a real checkpoint,
or other state. So we only upgrade the ID after we have reached
a checkpoint state during an active index run, and only after the
checkpoint has been confirmed.
Once a job has been upgraded and checkpointed, the version increments
and the new ID is used in the future. All new jobs use the
new ID from the start
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