IRAC: configure the tombstone cleanup task

Configure the frequency for the tombstone cleanup task.

The idea is to have a dynamic interval based on the target in memory tombstones. For example, if the user configures to store 100k tombstones in memory, after a cleanup task we check how many are there. If it is above this target, change the schedule interval to a smaller value; otherwise, we can increase the interval and run the task less frequently.