Add cache and persistence to registry

[gabriel-indik]

Two items were discussed this morning:

• Adding caching to the identity registry
• Adding persistence to the identity registry

This PR addresses these items.

[dwertent]

To prevent unintended changes to the cached data, this function should return a list of objects after RegistryNodeTransportEntry objects after running a deepCopy. Otherwise, the caller can make changes and that will change the cached data.

[gabriel-indik]

Thank you for the feedback, great point.
Would this be a good way to resolve the issue @dwertent:

func (r *registry) getNodeTransports(node string) []*components.RegistryNodeTransportEntry {
	re, _ := r.registryCache.Get(node)
	cpy := make([]*components.RegistryNodeTransportEntry, len(re))
	copy(cpy, re)
	return cpy
}

[dwertent]

I don't think that's the case, as the copy function performs a shallow copy by default. You can verify this with a simple test like the following:

r.UpsertTransportDetails(...)
n1 := r.getNodeTransports(<node>)
n2 := r.registryCache.Get(node)

for i := range n1 {
    assert.NotSame(t, n1[i], n2[i]) // compare pointers
}

To fix this, you can either implement a DeepCopy function, like so:

func (r *registry) getNodeTransports(node string) []*components.RegistryNodeTransportEntry {
    re, _ := r.registryCache.Get(node)
    cpy := make([]*components.RegistryNodeTransportEntry, len(re))
    for i, reg := range re {
        cpy[i] = reg.DeepCopy() // this would need to be implemented
    }
    return cpy
}

Alternatively, you could use a shallow copy method for structs without pointers:

func (r *registry) getNodeTransports(node string) []*components.RegistryNodeTransportEntry {
    re, _ := r.registryCache.Get(node)
    cpy := make([]*components.RegistryNodeTransportEntry, len(re))
    for i, reg := range re {
        tmp := *reg
        cpy[i] = &tmp
    }
    return cpy
}

However, the second solution won't work if RegistryNodeTransportEntry contains fields with pointers.