fix(everything) log.Debug -> log.Debugf

cc @whyrusleeping

perf(core/datastore) disable snappy compression

blockservice: dont write blocks twice

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