doh! I forgot to make sure leveldb is only opened once. thanks for catching this @mappum

* You may be wondering why we don't just share pointers to FSRepos. We
  want to manage the lifecycle of the FSRepo by tracking its `state`.
  Thus each FSRepo/goroutine requires private instance variables. For
  this reason, each `fsrepo.At(p)` caller must get its own goroutine.

* There's a test in `fsrepo` because callers desire the ability to Open
  from multiple goroutines. There's a test in `component` because this
  is where the actual work needs to go in order to provide the desired
  contract. If the `component` package moves, the assurances need to
  move along with it.

cc @whyrusleeping @jbenet

side note: there are a couple packages in FSRepo that it might be
worthwhile to extract once the dust settles on this feature-set.