Throttle: add a simpler throttle that just blocks above a threshold

Throttle: add a simpler throttle that just blocks above a threshold

This is convenient to use to turn synchronous calls into asynchronous
calls with a limited number of operations in flight concurrently.
It assumes the caller will wait for all operations to complete at the
end.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>