voyager transaction batch plugin improvements

[benluelo]

currently the transaction batch plugin works essentially like this:

1. N messages to be sent
2. batch those into B-sized chunks
3. for all of these, generate one light client update
4. once the lc update is generated, fetch proofs for all of the messages at the height of the light client update
5. submit all messages as a conc(submit(...updates, ...B1), seq(waitForClientHeight(), submit(...Bn)))

this has a few issues:

• if the first message fails (for whatever reason), then the update will never go through. this will cause the trailing messages to spin forever, unless a client is updated to exactly the same height as the update they were expecting (not likely).
• similar to the first point, and put more generally, this introduces a dependency on the first message for all of the subsequent messages.

an ideal design would instead at step 4 only fetch proofs for the items in the first batch (B1), and then for the rest of the messages queue them as seq(waitForAtLeastClientHeight(), mkMsgs(...Bn)). mkMsgs would then query the latest client state height, and generate proofs at that height instead.