Compaction time increase

Full compaction time (min), 4 nodes, 1B x 1KB, 15K ops/sec (90/10 R/W), 5% cache miss rate, s=1 c=1000

http://showfast.sc.couchbase.com/#/timeline/Linux/kv/compact/all

7.1.0-2021 - http://perf.jenkins.couchbase.com/job/athena/3096/ - 82.8

7.1.0-2123 - http://perf.jenkins.couchbase.com/job/athena/3164/ - 215.8

http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=athena_710-2021_compact_de77&snapshot=athena_710-2123_compact_9197