publish-subscribe should return an async channel

[stchang]

[stchang]

get-reply-evt, implemented by @m4burns, accomplishes something similar

[stchang]

@m4burns, can you check out the cleanup branch if you get a chance?

Your code will break in the following ways (let me know if it's too painful and I'll change it back):

1. I flipped the arguments in connection-pool-lease and friends, to make the pool optional, to make it more Rackety.
2. get-reply and get-reply-evt takes a connection now instead of a port. I'd like to prevent access to the connection struct internals if possible.

Otherwise, I just did a lot of cleanup, like removing the current-redis-connection and related stuff.

Also I wrote some documentation. Let me know what you think.

Specifically, I'd like your feedback on the pubsub-specific connections and whether you think they'll be useful for typical use cases?