Network backend for runtime configuration

[rshriram]

Currently Envoy reads all runtime config info from the filesystem. Changes to runtime need to be done on a separate copy of the filesystem tree, followed by a symlink swap.

For certain deployment scenarios, it might be easier and flexible to pull runtime info from a REST/gRPC backend similar to the SDS API.

If one were to add a network backend to the runtime, in addition to a file system backend, how should the APIs look like?

[rshriram]

This relates to #298 . Lots of runtime configuration are provided as part of RDS api. Granted that there are also a lot of other runtime settings that are not in RDS. But I just wanted to bring up the RDS overlap.

[rshriram]

Closing for now. With RDS, CDS, and SDS, adding another network work backend for runtime seems to overcomplicate the system. Its far easier to live with a file-system based backend, as it makes it easier to reason about the state of the Envoy process.