liveness and stats endpoints

[alexhsamuel]

• Add /api/v1/live and /api/v1/stats endpoints, to monitor basic liveness and stats, respectively.
• Add estimate of async event loop latency to stats.

To check liveness, with a 15 s timeout:

>>> from apsis.service.client import Client
>>> Client().alive(timeout=15)

To peek at the current latency:

>>> Client().stats()["async"]["latency"]

I see typically ~3 ms when Apsis is idle.