Define Message custom parser interfaces

[pharr117]

The Transaction/Message main loop will need to pass the raw RPC data to custom parsers so that developers may extend the datasets parsed from the messages.

The parsers will require the following at the very least:

1. A method to associate the parser with a Message type
2. A parser method that is passed the raw RPC Transaction/Message data and returns custom datasets
3. An indexer method that:
   • Gets the DB connection
   • Gets the base indexed data for the Block, Transaction and the particular Message value

The intention of (3) is to pass the custom parser all the data it will need to insert its custom dataset into the database.