StreamSocketOutput.WriteAsync is Sync on Full framework (SSL)

[benaadams]

From #569; PR #588 resolves coreclr, however full framework still has the issue.

Issue on full framework if made sync (it blocks)

Fullframework uses framework's SSL stream the corefx SSL steam only supports

net46 (.NETFramework,Version=v4.6)
netstandard1.3 (.NETStandard,Version=v1.3)

So would currently need to version bump from v4.5.1 to use it or get the sslstream retargeted (if that would even work?)

[benaadams]

Being looked at dotnet/corefx#7800 (comment)

[benaadams]

See https://github.com/dotnet/corefx/issues/8358

[muratg]

Let's try this in RTM.