"sie" is the short for "system info encryption". it is a wrapper around
a `opts` map so we can get the individual option by providing a default
value via an `optional<>` return value. but "sie" could be difficult to
understand without more context. and it is used like a function -- we
get the individual option using its operator().
so, in order to improve the readability, in this change, we rename it
to "get_opt".
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
3b7a991f74