fix(bitswap) duplicate key in wantlist

@whyrusleeping noticed this a couple days ago

potential long-term fix: prevent duplicate entries in the wantlist by
using a map/set and iterating over this data structure on export
style(bitswap/message) rename AppendWanted -> AddWanted

implementation will be patched to ensure bitswap messages cannot contain
duplicate blocks or keys
refactor(bitswap/message) use map to prevent duplicate entries

A nice invariant for bitswap sessions:

        Senders and receivers can trust that messages do not contain
        duplicate blocks or duplicate keys. Backing the message with a
        map enforces this invariant.

        This comes at the cost of O(n) getters.
style(bitswap/message) rename struct

so there's one less name to think about
fix(bitswap/message) impl with map to ensure no duplicate blocks

comes at the cost of O(n) Blocks() method.
style(bitswap/message) rename method -> AddBlock

to emphasize idempotence
test(bitswap/message) no duplicates
fix(bitswap) preserve ordering in bitswap message
docs(bitswap/message) BitSwapMessage interface
Merge pull request #216 from jbenet/fix/bsmsg-duplicates

fix(bitswap/message) duplicate entries