Refactor SafeFreeCredentials class

[kapilash]

Tracking issue to fix the following code smells ``SafeFreeCredentials` class:

* As pointed out in the PR comments of dotnet/corefx#6469, this class needs to be split up into two separate classes (via dotnet/corefx#8452)

• Rework PAL to remove non- SafeHandle SafeHandles. (Source of this observation was the PR Comments of dotnet/corefx#6469 )

[kapilash]

First part is done via dotnet/corefx#8452. libssl has also been cleaned.

The second part , namely, reworking PAL to eliminate non-safe handle, safe handles is an involved change and considering the impact, is a riskier change to get into RTM at this point.
So moving the milestone of the bug to 1.1.0

[Priya91]

It's unclear what remaining work is left here. @kapilash Can you elaborate on the next steps. Thanks!

[Priya91]

Closing due to lack of response.