From Paweł:
This is another episode in the "convert X to streamed mutations" series.
Hashing mutations (mainly for repair) is converted so that it doesn't
need to rebuild whole mutation.
The first part of the series changes the way streamed mutations deal
with range tombstones. Since it is not necessary to make sure we write
disjoint tombstones to sstables there is no need anymore for streamed
mutations to produce disjoint tombstones and, consequently, no need for
range tombstones to be split into range_tombstone_begin and
range_tombstone_end.
The second part is the actual hashing implementation. However, to ensure
that the hash depends only on the contents of the mutation and no the
way it is stored in different data sources range tombstones have to be
made disjoint before they are hashed.
This series also ensures that any changes caused by streamed mutations
to hashing and streaming do not break repair during upgrade.
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