Faster starts, pauses and wakes

Runtime now keeps machines of the most common sandbox shapes started ahead of time, and the servers keep memory ready for sandboxes that start or grow at once. Measured through the public API: a new sandbox runs its first Python command 374 ms after the create request at the median (495 ms p95, down from 981 ms), a pause takes 207 ms, a paused sandbox runs its next command 980 ms after the call that wakes it, and a preview visit to one answers in 0.77 s, down from 3.99 s. See speed for every figure and the samples.