Fix possible deadlock in TaskRunner.collect_updates.
Fix possible deadlock in TaskRunner.collect_updates.

Testing Done:
Filed https://issues.apache.org/jira/browse/AURORA-709 to track adding test
coverage for this.

Bugs closed: AURORA-669

Reviewed at https://reviews.apache.org/r/25175/

apache/aurora,protochron/aurora,thinker0/aurora,shahankhatch/aurora,protochron/aurora,crashlytics/aurora,rosmo/aurora,mschenck/aurora,mschenck/aurora,protochron/aurora,thinker0/aurora,rdelval/aurora,wfarner/aurora,rdelval/aurora,crashlytics/aurora,shahankhatch/aurora,rdelval/aurora,rdelval/aurora,medallia/aurora,wfarner/aurora,wfarner/aurora,protochron/aurora,medallia/aurora,protochron/aurora,mschenck/aurora,shahankhatch/aurora,wfarner/aurora,rosmo/aurora,kidaa/aurora,kidaa/aurora,shahankhatch/aurora,crashlytics/aurora,thinker0/aurora,shahankhatch/aurora,apache/aurora,medallia/aurora,apache/aurora,kidaa/aurora,kidaa/aurora,rosmo/aurora,rosmo/aurora,apache/aurora,mschenck/aurora,mschenck/aurora,medallia/aurora,kidaa/aurora,kidaa/aurora,rdelval/aurora,mschenck/aurora,shahankhatch/aurora,crashlytics/aurora,protochron/aurora,medallia/aurora,rosmo/aurora,apache/aurora,thinker0/aurora,medallia/aurora,crashlytics/aurora,thinker0/aurora,wfarner/aurora,apache/aurora,crashlytics/aurora,wfarner/aurora,rosmo/aurora,thinker0/aurora,rdelval/aurora