transport/server: CQL unset value support
transport/server: CQL unset value support

This patch implements support for CQL unset values at the protocol level.

Fixes #2039

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