Akka.Quartz.Actor (add support for .NetStandard 1.6)

[alexvaluyskiy]

Add support for .NetStandard 1.6, at least create a PR with Quartz.NET 3.0 alpha

[kerbybarrett]

What is the use case here? Do you have a set of requirements?

[Aaronontheweb]

@kerbybarrett persistently scheduled reminders inside an Akka.NET cluster, for things like "do this once every 24 hours"... The built-in scheduler is ephemeral and designed for high frequency, not precise periods of time like Quartz.NET

[alexvaluyskiy]

@Aaronontheweb Quartz.NET still does not support .NET Standard, so we could remove it from the milestone