Added a high-priority queue to BufferedProducer to avoid message re-ordering#153

Merged

mfontanini merged 4 commits intomfontanini:masterfrom

demin80:hi-priority-queue-fix

Jan 15, 2019

Commits on Jan 7, 2019

Commits on Jan 8, 2019

Revised the implementation based on the reviewers' response
demin80
committed

Commits on Jan 10, 2019

refactored by adding retry_mutex_ and replacing bools with enums; fixed formatting issues
demin80
committed