dht fixes

TestProversMany still fails

odd behavior: only one dht (the last one) is seeing
changes to its routing table.

i made a separate function because we may want to update our
routing table based on "closer peers". maybe not-- these could
all be lies.

Withe queries (particularly providers), it was possible to
exit the query runner's Run BEFORE all its children were done,
because the runner itself only listened to the context. This
introduced the possibility of a panic (you can go check it
out by running the TestProvidersMany test on dht_test in commits
before this one).  Thankfully, ctxgroup saved the day with
almost _zero_ changes to the sync flow, and now we have the
guarantee that the query runner will only exit if all its
children are done. ❤

Conflicts:
	routing/dht/query.go

This commit introduces a reader and writer that respect
contexts. Warning: careful how you use them. Returning
leaves a goroutine reading until the read finishes.

The Peer is Dead. Long live the ID

@jbenet @whyrusleeping

This commit replaces peer.PeerInfo with peer.ID in the bitswap package

@jbenet @whyrusleeping

the next commit will change bitswap.Network.FindProviders to only deal
with IDs