R1:5671c4b27669

Make PullLocal daemon more flexible and transparent about scheduling

Summary:
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.