Turn off fragmentation based auto compaction for magma

Magma performs incremental auto compaction internally and does not require an external agent trigger to coordinate compactions. We can turn off auto-compaction permanently for magma. The UI option to perform manual compaction can be still kept as it is for debuggability purposes or performing full compaction on magma.

Magma reports data size as logical size estimated by adding up the document sizes before compression. If we run pure create workload, the on-disk size can be lesser than estimated logical data size. The default ns_server fragmentation computation uses the formula (disk usage - data size)/disk_usage. Since data size could be larger than disk size, we end up reporting -ve fragmentation during 100% create workload. We need to set fragmentation to zero if data size > disk usage.