Implement z_sendmany and async RPC calls.

[nathan-at-least]

Depends on #1199 for note tracking.

This will require several substantial changes:

• adding a "proving work queue" into the wallet, which executes proofs one at a time.
• adding status tracking for the work queue.
• adding the z_sendmany call which adds jobs to the queue.
• add status checking calls for checking on work queue status.

Excluded for simplicity:

• cancelling jobs in the queue.
• getting any kind of status information aside from queued / finished-ok / finished-error.

[daira]

#1367 has been merged.