dirty_memory_manager: add functions to manipulate real dirty
dirty_memory_manager: add functions to manipulate real dirty

There are times in which we want to add and remove real dirty memory
without impacting virtual dirty. One such example is the cache update
process, where real dirty is the limiting factor.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

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