pbdagreader: use FSNode instead of protobuf structure

[schomatis]

Addresses part of #5192.

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pbdagreader: use FSNode instead of protobuf structure

Focus on the UnixFS layer and avoid explicit references to protocol buffers
format (used to serialize objects of that layer). Use the `unixfs.FSNode`
structure which it abstracts from the `unixfs.pb.Data` format.

Replace `PBDagReader` field `ftpb.Data` with `ft.FSNode`, renaming it to `file`
(which is the type of UnixFS object represented in the reader) and changing its
comment removing the "cached" reference, as this structure is not used here as a
cache (`PBDagReader` doesn't modify the DAG, it's read-only). Also, removed
unused `ProtoNode` field to avoid confusions, as it would normally be present if
the `FSNode` was in fact used as a cache of the contents of the `ProtoNode`.

An example of the advantage of shifting the focus from the format to the UnixFS
layer is dropping the of use `len(pb.Blocksizes)` in favor of the more clear
`NumChildren()` abstraction.

Added `BlockSize()` accessor.

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unixfs: remove `Get` prefix from `FSNode` accessors

See https://golang.org/doc/effective_go.html#Getters.

[whyrusleeping]

Thats a bit nicer