sstables: stats: noexcept all accessors
sstables: stats: noexcept all accessors

Signed-off-by: Michael Livshin <ae60993c65de9cdf2ca07b7426c976808535d555@scylladb.com>

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