fix(routing/dht) _always_ close chan on exit of FindProvidersAsync

the important change here is that within FindProvidersAsync, the channel
is closed using a `defer`. This ensures the channel is always closed,
regardless of the path taken to exit.

+ misc cleanup

cc @whyrusleeping @jbenet

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>