add an initial implementation of a service_id structure and three

add an initial implementation of a service_id structure and three
controls to  register/unregister/check a server id.

a server id consists of TYPE:VNN:ID    where type is specific to the
application.  VNN is the node where the serverid was registered and ID
might be a node unique identifier such as a pid or similar.

Clients can register a server id for themself at the local ctdb daemon.
When a client dissappears   or when the domain socket connection for the
client drops  then any and all server ids registered across that domain
socket will also be automatically removed from the store.

clients can register as many server_ids as they want at the same time
but each TYPE:VNN:ID must be globally unique.

Clients have the option of explicitely unregister a server id by using
the UNREGISTER control.

Registration and unregistration can only be done by clients to the local
daemon. clients can not register their server id to a remote node.

clients can check if a server id does exist on any ctdb node in the
network by using the check control