mutation_partition: Use B-tree insertion sugar
mutation_partition: Use B-tree insertion sugar

The B-tree .insert methods accept unique pointers and release them

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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