Disk io library: fix thread IsOwned() method by adding missing const qualifier.
Chunk server: rework client thread locking. Ensure that the current client thread pointer set correctly (properly restored when required) by unlock and lock sequence (re-lock). Allow direct access to the client thread mutex only for sanity check / verification purposes by adding "const" qualifier to the return value of the corresponding accessor methods, and making client thread mutext available only for the main event dispatch thread where the value of the current client thread pointer must be null. Implement current client thread pointer validation with the main thread loop in order to catch possible invalid uses of the client threads mutex, and missing current client thread pointer set and restore.