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.
Merge branch '2934-limit-crunch-logs' of git.curoverse.com:arvados into 2934-limit-crunch-logs

Fixing broken rebase
2934: add launch_crunch_dispatch to integration test

Added functionality for launching crunch-dispatch in
crunch_dispatch_test.rb while integration tests are running.

Refs #2934.
2934: count events and bytes in-memory

To avoid thrashing the database, crunch-dispatch.rb counts log events
and bytes in RAM instead of reading the database each time.

Refs #2934.
2934: add "bar" repository to test fixture

Add the "bar" repository to test/fixtures/repositories.yml so
CrunchDispatchTest may create a job using a commit from that
repository. (Change lost in a bad rebase.)

Refs #2934
Merge branch 'master' into 2934-limit-crunch-logs

Refs #2934
2934: add crunch_dispatch_test repository

Move files for crunch_dispatch_test to their own test fixture
repository.

Refs #2934.
2934: document stderr messages for commit tests

Some unit tests for the Commit models issue alarming but expected git
error messages on stderr.  Added comments documenting these in
commits_controller_test.rb.

Refs #2934.
Merge branch '2934-limit-crunch-logs'

Closes #2934