PVGroups backed by actual databases (e.g., elasticsearch)

[klauer]

Add some "advanced" examples based on the concept of PVGroups backed by actual databases. May be worthy of additional 'helpers' in a separate repository.

Of one of the possible forms:

1. Existing database -> pull -> PVGroup.pvproperty
2. PVGroup as a document -> push -> database

A variant of (2) is prototyped here for elasticsearch https://github.com/pcdshub/archstats/blob/master/archstats/db_backed.py where:

1. Data is sourced from a simple http request generating a JSON document
2. ... which is then exposed to EPICS by way of a dynamic PVGroup
3. ... and simultaneously stashed away as a document in elasticsearch

[danielballan]

Wow, cool.