2934: add limits for crunch job log generation.

2934: add limits for crunch job log generation.

* crunch_limit_log_events_per_job
* crunch_limit_log_event_bytes_per_job

  Limit the number of events a job may log, and the total number of
  bytes a job may log.

* crunch_log_bytes_per_event
* crunch_log_seconds_between_events

  Set minimum logging thresholds: event logs will only be flushed to the
  database when at least crunch_log_bytes_per_event have been generated,
  or crunch_log_seconds_between_events have passed.

Integration test CrunchDispatchTest exercised creating a job.

Refs #2934.