cql3: change operation::make_* functions to static
cql3: change operation::make_* functions to static

This commit makes operation::make* functions static, because they
don't access any instance-specific data anyway. It is later needed
to decouple setter execution from binding a cql3::term.

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