R1:ebcab8edb69d

Namespace Aphlict clients by request path, plus other fixes

Summary:
Fixes T7130. Fixes T7041. Fixes T7012.

Major change here is partitioning clients. In the Phacility cluster, being able to get a huge pile of instances on a single server -- without needing to run a process per instance -- is desirable.

To accomplish this, just bucket clients by the path they connect with. This will let us set client URIs to `/instancename/` and then route connections to a small set of servers. This degrades cleanly in…