Resolved issues:

Partially tackles #4585

Description of changes:

This PR strengthens our assertion that servers don't use the config before the client hello callback is invoked (allowing the user to swap out the config based on information in the client hello.)
I added a test that we aren't dereferencing anything on the config before the client hello cb is invoked. The way I do this is by setting the connection's config pointer to null. Any attempt to dereference pointers on that config will error/segfault.

I fixed a couple places where we do reference the config before the callback is invoked. One of those is setting a default ecc curve if the client doesn't send any ecc preferences in the extensions. I moved that chunk of code from the "parsing" function to the "processing" function.

Call-outs:

I'm not fixing everything in #4585. Our sslv2 client hello processing code uses the config before the client hello callback is invoked. I don't want to refactor our sslv2 client hello code right now, as we don't yet have an integration test for that behavior. Once this is done, we should have more assurance that a refactor is safe.

Testing:

Adds tests.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.