Priority queue proposal (#1615)
Priority queue proposal (#1615)

* Add the first draft of priority queue proposal

* API changes and open questions addition in priority queue

Vedin/corefxlab,tarekgh/corefxlab,ahsonkhan/corefxlab,Vedin/corefxlab,stephentoub/corefxlab,KrzysztofCwalina/corefxlab,ericstj/corefxlab,Vedin/corefxlab,KrzysztofCwalina/corefxlab,ericstj/corefxlab,whoisj/corefxlab,stephentoub/corefxlab,VSadov/corefxlab,VSadov/corefxlab,ahsonkhan/corefxlab,tarekgh/corefxlab,VSadov/corefxlab,Vedin/corefxlab,Vedin/corefxlab,dotnet/corefxlab,stephentoub/corefxlab,Vedin/corefxlab,joshfree/corefxlab,benaadams/corefxlab,benaadams/corefxlab,joshfree/corefxlab,whoisj/corefxlab,adamsitnik/corefxlab,adamsitnik/corefxlab,dotnet/corefxlab,VSadov/corefxlab,ericstj/corefxlab,ericstj/corefxlab,stephentoub/corefxlab,VSadov/corefxlab,ericstj/corefxlab,stephentoub/corefxlab,VSadov/corefxlab,ericstj/corefxlab,stephentoub/corefxlab