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

@whyrusleeping @jbenet newly initialized datastores were being dumped
into ./go-ipfs. Eeek. An oversight during the FSRepo refactor. Not sure
how this bug survived this long.

p2p/net/conn: reuse listen tcp port to dial

This commit cleans up the reuse port setup, and fixes a problem:
make sure to filter addrs out that we simply cannot dial with
(e.g. loopback -> non-loopback, or linklocal -> nonlinklocal)