Improve address naming in aether/lock

[ecpeterson]

@karalekas is not thrilled with the [upward|downward]-[rx|tx]-latch(es)? names used in aether/lock. Consider improving these.

Here's a longer-form comment he left elsewhere:

I find this upward/downward rx/tx language kinda confusing -- definitely took me a while to grok. Once I drew a picture I understood why you chose that language, but I think there might be a better choice. I haven't zeroed in on something that I love, but it's clear to me that the downward-tx and upward-rx channels are really about the unlocking part. Here is my explanation of what each does:

downward-rx: the channels that a lock requester listens on for updates from its lock targets
downward-tx: the channels that a lock requester should send unlocks to when we're done with the critical section
upward-rx: the channel that a lock target listens on for an unlock notification
upward-tx: the channel for a lock target to send updates to its lock requester

There's nothing explicitly wrong about the current naming convention (obviously), but maybe requester/target is clearer than upward/downward. We also dont really use RX/TX that much in the application source -- we often use more verbose descriptions like "reply" or "listen". Finally, the word latch is fun and colorful but maybe it would just be better to be super plain, unless we invest in more documentation and visualization to accompany the language.