utils: add fragment range
utils: add fragment range

This patch introduces a FragmentRange concept which is the minimal interface all
classes representing a fragmented buffer should satisfy.

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