Remove old data from database

[kderme]

The db keeps a lot of old records not related to the currest state. While keeping the historic evolution of values can be useful for explorers, it also takes a lot of disk space. We should have an option to periodically prune old values similar to how we prune spent outputs. These values can include:

• old stake distribution
• old rewards
• old drep distribution
  ...

The insert options can be extended with a new field:
prune: {
epoch_stake:: Integer
rewards:: Integer
drep_distr: Integer
}

Where each Integer shows many many epochs in the past should be maintained (default would be null).