This doesn't make a performance difference, but even better, it
simplifies the code.
We hacked test_multirpc to send 200x as many commands, and timed the
pytest over 20 runs:
Before:
=================== 1 passed, 136 deselected in 8.550000-9.400000(9.0045+/-0.2) seconds ===================
After:
=================== 1 passed, 136 deselected in 8.540000-9.370000(8.97286+/-0.16) seconds ===================
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>