[Feature Request] Logging subsystem improvements

As we scale up our Kapacitor deployment we are incresingly seeing situations where a badly configured task or even corrupt data being sent to a good task can cause a huge flood of logging. The log subsystem should be developed to help us better manage and even avoid these situations.

Some proposals for enhancements:
- [ ] Ability to send logs of each task and topic to separate files
- [ ] Control of log level on a per-task basis
- [ ] Ability to enable log throttling and duplicate log line aggregation with configurable parameters.

No files, tests, or entry points are named. Start by locating Kapacitor’s logging subsystem and task/topic log handling, then clarify the scope of separate outputs, per-task levels, and throttling or duplicate-line aggregation. Done requires an agreed design and tests for the selected enhancements.

Written by the indexing model from the issue text.