fix(blockstore, bitswap) enforce threadsafety in blockstore

fixes data race detected in a testnet test
fix(bitswap) move mutex up to strategy from ledger

addresses concurrent access in bitswap session
Merge pull request #198 from jbenet/fix/bitswap-races

fix(bitswap) data races
refactor(dht/pb) move proto to pb package
fix(bitswap) rm todo
style(bitswap) import