Called from try_flush_memtable_to_sstable, maybe_wait_for_sstable_count_reduction will wait for compaction to catch up with memtable flush if there the bucket to compact is inflated, having too many sstables. In that case we don't want to add fuel to the fire by creating yet another sstable. Fixes #4116 Signed-off-by: Benny Halevy <bhalevy@scylladb.com>