compaction: share sstable set with incremental reader selector
compaction: share sstable set with incremental reader selector

By doing that, we'll be able to release exhausted sstable from both
simulteaneously.
That's achieved by sharing set containing input sstables with the incremental
reader selector and removing exhausted sstables from shared set when the
time has come.

Step towards reducing disk requirement for compaction by making it delete
sstable which all data is in a sealed new sstable. For that to happen,
all references must be gone.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

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