Extend custom parser interfaces to include pre-insertion cleanup methods to support reindexing

[pharr117]

The current custom parser direction requires devs to build their own indexer methods. This provides flexibility to allow custom datasets to be introduced. However, since we are using gorm with relational models, we will want to provide the interfaces with a pre-index callable method to allow cleaning up old datasets if they exist. This will be called before custom dataset insertions during reindex.

[pharr117]

Cancelling this one for now. Each custom parser interface for messages and blocks contains a Index* method that contains the entire parsed dataset. This method can be used to do pre-insertion cleanups if needed. We can revisit this if it becomes a burden to force the functionality to be implemented in the Index* method.