sstables: get rid of 64kB minimum index advance to generate summary
sstables: get rid of 64kB minimum index advance to generate summary

Limiting summary entry generation to at most one summary entry
per 64k of index data can lead to large index pages, with thousands
of index entries per summary entry. These are slow to parse, and there
is no real gain from the limit, since we already enforce a size
limit on the summary.

Remove the limit and allow summary entry generation based solely on
spanned data size.

Fixes #2711.
Message-Id: <50f9c9807a7ef848b891bb6f0bc9679fa45a5ece@scylladb.com>

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