Closes#10930
* github.com:scylladb/scylla:
test: perf_row_cache_update: Flush std output after each line
test: perf_row_cache_update: Drain background cleaner before starting the test
test: perf_row_cache_update: Measure memtable filling time
test: perf_row_cache_update: Respect preemption when applying mutations
test: perf_row_cache_update: Drop unused pk variable