Implement System.Threading.Thread.GetNewManagedID as an icall rather than in managed code, so that it is not reset by a domain reload.
Altered GetNewManagedId to forward to a _internal method instead of being an icall itself, and marked it as a safecall in the audit list
Merge pull request #169 from Unity-Technologies/managed-thread-id-fix

Managed thread id fix (case 601660)