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)

This commit finally makes use of the sent observed addrs.
If the connection's local address is from one of our
listen addrs, then the remote's observed addr is its
natted mapping, which is useful to us. For now, we add
it directly to our address book. (a future commit should
make addressbook addresses expire)

fix(bitswap.decision.Engine) send only the freshest messages

@whyrusleeping

Before, the engine worker would pop a task and block on send to the
bitswap worker even if the bitswap worker wasn't to receive. Since the
task could have been invalidated during this blocking send, a small
number of stale (already acquired) blocks would be send to partners.

Now, tasks are only popped off of the queue when bitswap is ready to
send them over the wire. This is accomplished by removing the
outboxChanBuffer and implementing a two-phase communication sequence.

fix fuse mounting issues

This commit moves the record validation/verification
from dht/ into the new record/ packaage. Validator object
-- which is merely a map of ValidatorFuncs -- with a
VerifyRecord

cc @whyrusleeping

this time, without loading the private key on every startup