There's no need to actually compact a sstable which is fully expired
and which deletion of all its data will not ressurect older data.
For that, a sstable will only be considered fully expired if it
doesn't contain data newer than its overlapping counterparts.
That way, there could be a false negative, but never a false positive.
Currently, a fully expired sstable would unnecessarily waste read
bandwidth of disk. This will help a lot time series workloads in
which data for a given time window is all deleted at once using TTL.
Fixes#2620.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
809b30c4a2