Add start_new_page() to compact_mutation_state
Add start_new_page() to compact_mutation_state

start_new_page() resets the limits to the current page's ones and
sets the _empty_partition flag so that the partition header (if the last
page finished inside a partition) will be reemitted.

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