Persist model definition for k-NN indices

[jmazanec15]

Description

For training, we create a template index in order to initialize components for each segment. Currently, we do not persist the parameters around the model anywhere other than the serialized model component.

This provides a poor user experience because users may forget what parameters a particular model has. Also, from a developers perspective, it makes it difficult to look up information about particular models for optimizations.

Proposed solution

Instead, we should serialize the model information with the model-metadata and in the model system index. This needs to be done in a backwards compatible way. This way, we can look up the model parameters for a particular model easily.

[jmazanec15]

Addressed in #1527