swarm: clean APIs

Pass all methods in async and on the io pool if they write into
the repository. Moreover, now MessageReceived guarantees to be
ordered if announced. e.g. if a parrallel merge and commit are done
previously if the merge was done before the commit you could receives
MessageReceived(merge) after MessageReceived(commit). Now it's
impossible. So the linearizedParent is guaranteed to be announced
before.

Change-Id: If4b94f19f5bf5abd4841fc19074231e3411e077d