Ref T4605. Fixes T3466. The major change here is that we now run up to four simultaneous updates. This should ease cases where, e.g., one very slow repository was blocking other repositories. It also tends to increase load; the next diff will introduce smart backoff for cold repositories to ease this.

The rest of this is just a ton of logging so I can IRC debug these things by having users run them in phd debug pulllocal mode.

For T3466:

• You now have to hit four simultaneous hangs to completely block the update process.
• Importing repository updates are killed after 4 hours.
• Imported repository updates are killed after 15 minutes.

• Ran phd debug pulllocal and observed sensible logs and behavior.
• Interrupted daemon from sleeps and processing with diffusion.looksoon.
• Ran with various --not, --no-discovery flags.