mutation_partition: Use multi-line printout
mutation_partition: Use multi-line printout

Convert to a multi line output, which is easier to read for a human.

After:

{ks.cf key {key: pk{000c706b30303030303030303030}, token:-2018791535786252460} data {mutation_partition: {tombstone: none},
 range_tombstones: {},
 static: cont=1 {row: },
 clustered: {
    {rows_entry: cont=true dummy=false {position: clustered,ckp{000c636b30303030303030303030},0} {deletable_row: {row: }}},
    {rows_entry: cont=true dummy=true {position: clustered,ckp{000c636b30303030303030303031},0} {deletable_row: {row: }}}}}}

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