Bitswap rounds

This PR implements the first revision of 'Bitswap Rounds' as defined by @jbenet in issue #417 
The other case is when the datastore is (closed|unavailable|not okay).

https://github.com/jbenet/go-datastore/blob/master/Godeps/_workspace/src/github.com/syndtr/goleveldb/leveldb/db_state.go#L197