organize imports
privatize getLedger method
substitute with LedgerMap type
hide ledger wantlist manipulation
privatize fields in ledger
fix(bitswap:ledger) race conditions

https://github.com/jbenet/go-ipfs/issues/39
Merge pull request #38 from jbenet/feat/bitswap-import-cleanup

chore(bitswap) tidy up bitswap, ledger