Add thread safe variants of viper.Set* functions

[marcintustin]

Preflight Checklist

• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

Like the people complaining in this thread, I just found out the viper.Set* methods are not thread safe. #268

Proposed Solution

Add corresponding viper.SetSafe* methods that use a mutex to control access to the global object

Alternatives Considered

Deprecate the global instance.

Write really explicit documentation that these viper.Set* functions are not thread safe.

Additional Information

I'm happy to work on an MR once (if) the approach is agreed.