Link Compute Objects by ID

The ``ComputeNode``, ``Service``, and ``Instance`` objects form the primary
data model for our compute nodes. Instances run on compute nodes, which are
managed by services. We rely on this hierarchy to know where instances are
(physically) as well as which RPC endpoint to send messages to for management.
Currently, the linkage between all three objects is a relatively loose and
string-based, association using the hostname of the compute node and/or the
``CONF.host`` values. This not only makes an actual/intentional rename very
difficult, but also risks breaking critical links as a result of an
*accidental* one.