The set contains 3 small optimizations:
- avoid copying of partition key on lookup path
- reduce number of args carried around when creating a new entry
- save one partition key comparison on reader creation
Plus related satellite cleanups.
* https://github.com/xemul/scylla/tree/br-row-cache-less-copies:
row_cache: Revive do_find_or_create_entry concepts
populating reader: Do not copy decorated key too early
populating reader: Less allocator switching on population
populating reader: Fix indentation after previous patch
row_cache: Move missing entry creation into helper
test: Lookup an existing entry with its own helper
row_cache: Do not copy partition tombstone when creating cache entry
row_cache: Kill incomplete_tag
row_cache: Save one key compare on direct hit
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