Two ladders of kernel rungs

The kernel efficiency arc lands as numbered rungs, each on its own pull request. In #2381, rung 4, the EE thread blocks for an RPC reply instead of spinning. In #2418, rung 9, the IOP host sleeps to its true next deadline: idle, it woke about 695 times a second, 68 percent of them finding nothing, at a one millisecond seed that is now one 60 Hz field. In #2419, rung 10, the listener's socket write leaves the EE thread for a sender thread and a 1 MiB ring allocated once. #2457 folded the first ladder into the renderer branch, and #2555 carried rungs 14, 16 and 15 of the second.