Patch 55a8421e3d fixed an inefficiency when rebuilding
statistics with many compaction groups, but it incorrectly removed
the update for newly added SSTables. This patch restores it.
When a new SSTable is added to any of the groups, the stats are
incrementally updated (as before). On compaction completion,
statistics are still rebuilt by simply iterating through each
group, which keeps track of its own stats.
Unit tests are added to guarantee the stats are correct both after
compaction completion and memtable flush.
Fixes#12808.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Closes#12834
d6fe99abc4