fsrepo: Refactor to extract datastore internals

blocks/blockstore/blockstore.go

core/builder.go

core/core.go

core/corerouting/core.go

pin/pin.go

repo/fsrepo/defaultds.go

repo/fsrepo/fsrepo.go

repo/mock.go

repo/repo.go

routing/dht/dht.go

routing/none/none_client.go