memtable: Destroy partitions incrementally from clear_gently()
memtable: Destroy partitions incrementally from clear_gently()

Destroying large partitions may stall the reactor for a long
time. Avoid this by clearing incrementally.

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