Preemptively launch STUB+ circuits

See the TODO in hspool.rs

                // TODO HS-VANGUARDS: we will need to launch N STUB circuits and M STUB+
                // circuits, for some N, M.
                //
                // We will need Pool to have two different targets, one for STUB circuits and
                // another for STUB+. Otherwise, if we only know the overall circuit target, each
                // time the pool is low on circuits we'll have no choice but to spawn both kinds of
                // stub circuits (in a ratio of N/M), even if we don't necessarily need the deficit
                // to be replenished in the N/M ratio. IOW, if the pool's overall target number of
                // circuits is T = N + M, and the actual number of circuits in the pool is
                // L = T - D, we'll need to spawn D circuits that consist of X STUBs and Y
                // STUB+s, where X/Y is not necessarily N/M (but the overall STUB/STUB+ ratio
                // *is* N/M).