[WIP] Support structured logging

[tmds]

new public API:

class JournalMessage : IDisposable
{
   bool IsEnabled { get; }
   JournalMessage Append(string name, object value);
}
enum LogFlags { .. }
class ServiceManager
{
   static bool IsJournalAvailable { get; }
   static string SyslogIdentifier { get; set; }
   static JournalMessage GetJournalMessage();
   static void Log(LogFlags, JournalMessage);
}

Library changes from netstandard2.0 to netcoreapp2.0.