Reduce memory footprint when many instances of stat_entry_ema_rate share the same configuration. #3496
Cache the result when computing the ema alpha value.
Allow per-user I/O stats to be suppressed with INDIVIDUAL_TRANSFER_IO_REPORT=false.
Document all the I/O stat knobs.

This also fixes an uninitialized variable and a possible division by zero.
Merge branch 'V7_9_5-branch'