Allow vnode init to happen in parallel

Added an extra state to riak_core_vnode to decouple vnode initialization
from process creation.
Added a function to block until initialization is truly finished.
Vnode init call now takes a list of indices or single index.
The list version creates vnode processes first, then waits for them to
initialize, which allows it to happen in parallel.
Used the above in riak_core_ring_handler when starting services
on first ring event.
Tests show I/O saturation at bitcask startup now, instead of the
serialized trickle we had.
This code still needs some work to handle edge cases, specially around
how to handle vnode initialization failing.
Add concurrency control to vnode initialization

Also, a bit of code cleanup.
New pmap with bounded concurrency utility added with unit tests.
Fix cluster into to query vnode manager, not sup

With the parallel vnode change, there is more reason to not query the
supervisor directly, as its children may not have finished
initialization yet.
Merge pull request #274 from basho/eas-parallel-vnode-init

Allow parallel vnode initialization