Add ReSharper external annotations to log methods in the Akka assembly

This makes ReSharper understand that for example log.Error's first parameter is a format string and that the method behaves just like string.Format()

For more info, see
https://www.jetbrains.com/resharper/webhelp/Code_Analysis__External_Annotations.html
https://www.jetbrains.com/resharper/webhelp/Code_Analysis__String_Formatting_Methods.html