Add simple healthcheck

[slominskir]

Fixes #23

[slominskir]

Need to consider some user requests for PV that don't exist are possible. Should not trigger unhealthy. If never connected, then we can't readily know if because user requested PV really doesn't exist, or backend IOC is temporarily unavailable. If PV previously connected (has non null Last Updated Timestamp), then we can reasonably guess when something goes wrong.

[slominskir]

Simple Healthcheck Known Limitations:

• Won't catch issues specific to CaGet endpoint.
• All-or-nothing - if a single IOC breaks, and hosted PV previously being monitored, entire epics2web host marked unhealthy
• NeverConnected PVs are ignored, even though possible a connectivity issue. We can't tell if bad user request (non-existent PV) vs connectivity issue in this case, so we do nothing.