Instances of `variable_specifications` are passed around as
shared_ptr's, which are redundant in this case since the class
is marked as `final`. Use `lw_shared_ptr` instead since we know
for sure it's not a polymorphic pointer.
Tests: unit(debug)
Signed-off-by: Pavel Solodovnikov <pa.solodovnikov@scylladb.com>
Message-Id: <20191225232853.45395-1-pa.solodovnikov@scylladb.com>