MINOR: guid: introduce global UID module

MINOR: guid: introduce global UID module

Define a new module guid. Its purpose is to be able to attach a global
identifier for various objects such as proxies, servers and listeners.

A new type guid_node is defined. It will be stored in the objects which
can be referenced by such GUID. Several functions are implemented to
properly initialized, insert, remove and lookup GUID in a global tree.
Modification operations should only be conducted under thread isolation.