memtable: don't use encoding_stats epochs as default
memtable: don't use encoding_stats epochs as default

Why default to an artificial minimum when you can do better
with zero effort? Track the actual minima in the memtable instead.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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