Add logging to stderr for slow tiles (get/put) during a tilelive-copy

[yhahn]

• Always: changes getTileRetry and putTileRetry util functions to take a stream object and emit slow events when a tile op exceeds the slowTime threshold. slowTime threshold defaults to 60s.
• Optional: if options.slow = <time in ms> is passed to tilelive.copy(), copy sets the threshold and adds handlers for outputting to stderr whenever the slow event is emitted.
• Exposes this option to tilelive-copy bin.

This would start 5.10.x

@rclark 👀 ?