Remote Key Value store Support

[tanishq-dubey]

Key-Value stores such as Consul and Etcd are quite popular and used to store configuration, and I think they should be supported by gestalt. However, there are a few questions to ask before work can begin:

1. What priority should the KV be given?
2. Should the KV be read into memory at init time? Should the KV cache the values (only go to server on first call)? Or, should the KV be obtained from the server each time?
3. As a V1, what KV features should be implemented?

I think using the Consul and Etcd libraries should provide 90% of the functionality, but the goal should be to try and obfuscate complexity from the user, such that they only have to provide connection details, and then use gestalt as normal.

[tzachshabtay]

Maybe you can give control (for 1 & 2) to the user to decide which "config sources" exist and their priorities and also allow configuring how to behave internally. So, as an example, something like:

gestalt.set_sources(
              gestalt.JsonFile(),
              gestalt.EnvVars(),
              gestalt.Consul(cache=gestalt.NeverCache)
)

So in this example Consul is last in priority, and it will go to the server each time.