Add overloads for execute_internal and friends
accepting a vector of optional<data_value>.
The caller can pass nullopt for any unset value.
The vector of optionals is translated internally to
`cql3::raw_value_vector_with_unset` by `make_internal_options`.
This path will be called by system_keyspace::update_peer_info
for updating a subset of the system.peers columns.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>