consistency, cpu vs heap

Reorder cli commands so like is next to like, use same naming pattern
for cpu and heap files. I chose node.PID.[cpuprofile|heapsnapshot],
but I'm OK with `-` instead of `.`, and with adding a Date, as long
as both commands use same convention.

This is more of usability feedback to @kraman via PR than something I intend to finish... I just noticed the inconsistency of naming when integrating into pmctl. Thats a problem with PR review via diff... you don't notice the pre-existing code around it.

Thoughts?