Provider Strategies

[Stebalien]

@magik6k just added reprovider strategies to prevent nodes from reproviding for every block (unless configured to do so) at regular intervals. Unfortunately, we still publish provider records for every block as we add them. This adds a lot of work to ipfs add.

At a minimum, we should apply the reprovider strategies to initial provides. However, given that we know extra semantic information about the DAG at this point, we could replace/augment these the existing strategies with a smarter one that provides the root nodes of all added files/directories (we'd probably want to record this information in some provider database that gets "cleaned" every reprovide).

[Stebalien]

Duplicate of #3813.