Generate a callsite identifier associated with each callsite

1. Generate a unique string name for a callsite that appears in every node
2. This identifier will refer to a single callsite and is used for trace data retrieval

@ke-yu
@lukechurch
@Steell
@Benglin

.NET guideline:http://msdn.microsoft.com/en-us/library/dn169389(v=vs.110).aspx
 "Do not use Hashtable or Dictionary<TKey, TValue> in public APIs...public APIs should use IDictionary or IDictionary<TKey, TValue>"