Implement Block Event model indexing

[pharr117]

Each block can contain Begin Blocker and End Blocker events. See here for what they are used for.

We need to implement a model that will be used for indexing begin block and end block events.

Events in the begin block and end block sections of the block data are typical Cosmos (Tendermint) K/V pairs. See here for how they work.

We will want to store the values in the Block Events in the following way:

1. BlockEvent model
2. Key column
3. Value column

The best index will most likely be on the Key column.

[pharr117]

The above description was slightly incorrect due to some mistypes. The new models are:

1. BlockEvent - A model to store the main event emission
2. BlockEventType - Holds unique indexed block event types to reduce database size with these (usually) unique keys
3. BlockEventAttribute - Holds attributes linked to BlockEvents with a BelongsTo relation
4. BlockEventAttributeKey - Holds unique indexed block event attribute keys to reduce database size with these (usually) unique keys