Miscellaneous changes

The area above the lock was getting big. Moving this up to avoid
mistakes down the road.

closes #394 https://github.com/jbenet/go-ipfs/issues/394

initial implementation of repo-gc command

fix #424: when adding blocks, provide to exchange asynchronously

@jbenet

would like it to work this way

@jbenet

p2p/net/conn: only reuseport if avail

benchmark CLI `ipfs add`

races: fix race conditions