Continuous background profiling

[lucko]

• Continuously profile in the background to pick up historic performance issues
• Use a low sampling interval to reduce impact on server
• Opt-in, off by default (??)

Challenges

• Memory usage - needs to be fairly lightweight, can we periodically serialize & write to disk?
• Discarding old data
• How should the data be presented?
• Chunk data into ~5min blocks to allow for timings-like graph?