[#327] Default local `ServiceInstance` to a fixed `URI` i.o. `null`

[smcvb]

This pull request defaults the ServiceInstance of the AbstractCapabilityDiscoveryMode to a private ServiceInstance with a fixed URI.

Doing so, we prevent a scenario when the CapabilityDiscoveryMode#updateLocalCapabilities is never invoked, leading to a null ServiceInstance. Preventing this scenario ensures that Axon Framework instances that have zero command handlers will not fail on the "is this the local node I am asking capabilities for"-check, when retrieving the capabilities of other instances.

This pull request resolves #327

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