Expose Magma's filesystem based Read/Write bytes stat in kv_engine and fix seqIterator stats

Currently Magma only exposes NReadBytes from Magma stats a drawback of that stat is that it is delayed ie. it is updated only once an operation/iterator is completed. That means in case of longer running backfills where the sequence iterator is open, Magma read stats does not have live read bytes as seen in CBSE-13744. This makes it hard to analyse the rate of reads occurring.

Magma also tracks the read and write bytes at the filesystem layer via MagmaFileStats.TotalBytesRead but this is not unpacked in kv_engine as a separate stat. We should ideally use these for NRead/WriteBytes