refactor blockstores

[raulk]

• Place all blockstores into a single flattened package. Subpackages are only created for blockstores that depend on third-party components (e.g. badger).
• Make the blockstore.Blockstore interface the union of the basic IPFS blockstore + Viewer, for cleanness and to remove type assertions. This interface will be enhanced soon with a Sync() method: 5057.
• Introduce a blockstore.Adapt(BasicBlockstore) method to adapt a basic IPFS blockstore into a Lotus blockstore.
• Rename blockstores for consistency.
• Add godocs to all blockstores. Still TODO.

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Once we merge this, we'll be in a better position to add support for the Sync() method (#5057), and to continue extending the blockstore with specialised operations for better efficiency.

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Partly addresses #5455.

[raulk]

This depends on #5425.

[magik6k]

Just 2 minor nits

[raulk]

@magik6k should be ready to put on a boat. @vyzo and I are preparing a bunch of PRs for the splitstore on top of this.