Numerous fixes, fetch all records, added wait and retry using Polly and updated .NET versions

[mohsinonxrm]

@davidyack , please review and merge.
Fixes:
#49
#79
#81
#83
#86

[davidyack]

I'm worried about impact of the following to existing users - thoughts?
@mohsinonxrm
upgraded packages in all projects, targetting .net standard 2.0 only,… …
a50110a
@mohsinonxrm
updated dotnet core project to 3.1, updated standard to netstandard2.0 …

[mohsinonxrm]

@davidyack , if we revert the versions, then Polly won't work for resiliency. I've just implemented very basic logic for that. If this gets merged, I plan to enhance it further.

.net core 1.0 reached EOL last year, and the only two versions that are supported are 2.1 and 3.1 on LTS:

Moving to .net core 3.1 allows us to remove restrictions from the methods that aren't supported, and we can use Polly so that it won't limit the users but give them additional functionality.

Regarding .net standard 2.0, so it gives us more APIs and so allows us to remove restrictions from the methods the same as .net core. However, it does drop .net core 1.0 and 1.1 and some mono and Xamrin versions.

Here is my opinion; I think we should split the releases. After you're done reviewing and testing this PR and merge it, release it as a v2.0 version and maintain the current branch as v1.x, so that it can be supported for however long, it can be.

That way, work can continue on the v2.0 branch.

Thoughts?

[Sylistron]

What's the story here? Would love to have Polly support b/c we see 429s a lot...

[mohsinonxrm]

@Sylistron , well, I could fork and maintain my own version of it, perhaps?