compaction: size_tiered_compaction_strategy: get_buckets: consider only current bucket for each sstable
compaction: size_tiered_compaction_strategy: get_buckets: consider only current bucket for each sstable

Since the sstables are sorted in increasing size order
there is no need to consider all buckets to find a matching one.

Instead, just consider the most recently inserted bucket.

Once we see a sstable size outside the allowed range for this bucket,
create a new bucket and consider this one for the next sstable.

Note, `old_average_size` should be renamed since this change
turns it into a reference and it's assigned with the new average_size.
This patch keeps the old name to reduce the churn. The following
patch will do only the rename.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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