managed_ref: add get() accessor
managed_ref: add get() accessor

While a managed_ref emulates a reference more closely than it does
a pointer, it is still nullable, so add a get() (similar to
unique_ptr::get()) that can be nullptr if the reference is null.

The immediate use will be mutation_partition::_static_row, which
is often empty and takes up about 10% of a cache entry.

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