4266: Fix log processing performance.

4266: Fix log processing performance.

* Remove "max log table entries per job" limit (and its horrible
  memory leak).

* When logs are coming fast, but under all throttle thresholds, group
  them into fewer rows in the logs table.

* Optimize performance by making a short code path when any threshold
  is exceeded. Now capable of ~200 MiB/s (of 100-character lines) on
  lappy386.

* Bring stream buffer back up to a reasonable size, add a comment to
  protect it from future style tweaks.

* Simplify time calculations by storing time-to-reopen-throttle
  instead of time-throttle-last-opened.

* Log "# bytes skipped" to stderr (sysadmin logs) too, not just the
  logs table.