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

perf(cmd/ipfs) write mprof file in debug mode

TODO add memory profiling to ipfs2

discovered by @mappum and applied at https://github.com/jbenet/go-ipfs/commit/414e20b71e50b65e40883f3ac85f91aa81c1e486

fixing here to address #233

https://github.com/jbenet/go-ipfs/issues/233

refactor(blockservice, merkledag, namesys) deprecate u.ErrNotFound