commitlog: make discard_completed_segments and friends noexcept
commitlog: make discard_completed_segments and friends noexcept

To simplify table::seal_active_memtable error handling
and retry logic.

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

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