IRAC batch tombstone cleanup command

For performance improvement, we can batch keys in a single request to clean up the tombstones.

There are 2 cases where batch makes sense:

• when sending to the remote sites for checking if the tombstone can be removed
• when removing from the backup owner in the local cluster

To avoid introducing a "batch-size" configuration, reuse the value from the state transfer "chunk-size" (only keys are sent so this should be safe)