sstables: Make index_entry copyable
sstables: Make index_entry copyable

Needed to make the index_list copyable, which is going to be needed to
implement legacy get_index_entries() which returns by value, after
index sharing is implemented.

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