types: add a type for thread IDs and try to use it everywhere
diff --git a/src/base/types.hh b/src/base/types.hh
index 6d8d309..1a6db9f 100644
--- a/src/base/types.hh
+++ b/src/base/types.hh
@@ -68,4 +68,10 @@
 
 const Addr MaxAddr = (Addr)-1;
 
+/**
+ * Thread index/ID type
+ */
+typedef int16_t ThreadID;
+const ThreadID InvalidThreadID = (ThreadID)-1;
+
 #endif // __BASE_TYPES_HH__