improvements for stream stat logging refs T37552

- task thread time is shown as percentages of source, output, and
  filtering to facilitate performance analysis
- fix race conditions that could drop events
- dropped bundles (due to filters) are shown as a percentage
- in/out/dropped are shown per-interval, per-minute, and as totals
- numbers are formatted with commas
- some columns use minimum width formatting to reliably align
  columns vertically
- perf. optimizations for StreamMapper and MapFeeder -- far less
  redundant or useless Metrics objects
- other general clean-up