I would suggest having a separate option for defining the maximum number of vnodes started in parallel rather than re-using the existing `vnode_rolling_start` option. Esp. since the `vnode_rolling_start` option is still used for the existing rolling start logic which has nothing to do with starting primary vnodes. I could conceivably see folks wanting different values for these two different concepts.

To recap how Riak operates, when a node first comes online it must start up all primaries vnodes before marking a service as up. This code path is handled in `riak_core_ring_handler` and is the thing that calls `start_vnode` and therefore this newer parallel init code. After starting all primaries, the service is marked up and Riak starts operating as normal. However, Riak also starts all secondary vnodes at least once per node restart, to trigger any potential hinted handoff. This is handled by the rolling vnode start logic in `riak_core_vnode_manager` (see: `maybe_start_vnodes`) that starts up to `vnode_rolling_start` vnodes every vnode manager tick (with a frequency determined by `vnode_management_timer` config). Likewise, this code path starts vnodes sequentially, and doesn't use the new parallel vnode logic at all (it would be great to change rolling start logic to use parallel init as well, but I'd consider that out-of-scope for this PR).