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License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>

cc @whyrusleeping

If the datastore has a value for the key, we already have
the block. We should not write it again. This will make redundant
writes much faster.

At the moment, a datastore.Has on leveldb is a GetBackedHas.
Track https://github.com/jbenet/go-datastore/issues/6

the u.Hash error can be safely ignored (panic) because multihash
only fails from the selection of hash function. If the fn + length
are valid, it won't error.

cc @whyrusleeping

(perhaps should be logged instead)

examples:

    http://golang.org/pkg/container/heap/#Interface

    http://golang.org/pkg/net/#Interface

    http://golang.org/pkg/sort/#Interface

Move go-ipfs/bitswap package to go-ipfs/exchange/bitswap

* Delineates the difference between the generic exchange interface and
  implementations (eg. BitSwap protocol)

  Thus, the bitswap protocol can be refined without having to overthink
  how future exchanges will work. Aspects common to BitSwap and other
  exchanges can be extracted out to the exchange package in piecemeal.

  Future exchange implementations can be placed in sibling packages next
  to exchange/bitswap. (eg. exchange/multilateral)