compaction: scrub/segregate: adjust partition-estimate as buckets accumulate
compaction: scrub/segregate: adjust partition-estimate as buckets accumulate

Scrub compaction in segregate mode can split the input sstable into as
many as hundreds or even thousands of output sstables in the extreme
case. But even at a few dozen output sstables, most of these will only
have a few partitions with a few rows. These sstables however will still
have their bloom filter allocated according to the original
partition-count estimate, causing memory bloat or even OOM in the
extreme case.
This patch solves this by aggressively adjusting the partition count
downwards after the second bucket has been created. Each subsequent
bucket will halve the partition estimate, which will quickly reach 1.

Fixes: #9463

Closes #9464

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla