Fall back to TLSv1.2 when System.Security.Authentication.SslProtocols.None is disabled/unavailable

[michaelklishin]

Proposed Changes

See #764 for the background. In some environments, we cannot assume
that SslProtocols.None will behave as expected:

• It can be disabled via app context and other means
• Its effective behavior is different between .NET versions according
  to .NET community blog posts and GitHub issues

So we borrow from [1] and special case this exception to retry with an
explicitly defined version (TLSv1.2, same as modern .NET default).

The discussion in [2] should also be mentioned as they have led us
to a much better understanding of many aspects of this intricate setting.

Types of Changes

• Bug fix (non-breaking change which fixes issue 6.0: TLS connection fails when SslProtocols.None is unavailable or effectively disabled #764)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

Further Comments

Should address #764.

1. mysql-net/MySqlConnector@715c3b5
2. ArgumentException when using SslProtocols.None robinrodricks/FluentFTP#452 (comment)

[bording]

FYI there will be some conflicts between this and #763 because I've made SslHelper internal and moved the file to to the impl folder.

[bording]

The dream of eradicating "SSL" everywhere we can over a decade
after the name has been deprecated is still alive, though.

@michaelklishin Since we're already making breaking changes for 6.0, we could consider just renaming this class and dropping "SSL".

[michaelklishin]

Given that 6.0 introduces substantial resource footprint improvements, I'd like the upgrade to it to be as straightforward as possible, so renaming can wait. "SSL" in naming is probably not something many care about anyway.

[michaelklishin]

@bording I suspect this PR will go in first as it is arguably a bug fix. I'm afraid some adjustments ot your PR would be necessary. I can do them if you want.

[bording]

@bording I suspect this PR will go in first as it is arguably a bug fix. I'm afraid some adjustments ot your PR would be necessary. I can do them if you want.

I can make the adjustments.