switch to margo_thread_yield in sparkline fn
Fixes #53 (closed).
Three changes: use margo_thread_sleep in sparkline data collection function, release timers when margo is shutdown, and adjust progress loop to allow mercury to (safely) block when sparkline ult is present.
@sramesh this should make your profiles reflect normal behavior a little better because it won't force margo into busy spin mode unless it was configured that way already.
@mdorier can you give this a quick sanity check and merge the MR?