Add shared state for environment document between edge proxy instances

[matthewelwell]

Currently each Edge Proxy instance maintains a local cache of the environment document, and polls the core API itself.

As part of this issue, we'd like to centralise the environment document, ideally without sacrificing performance.

This issue is closely related to a couple of other issues:

• Feature request: streaming/SSE updates to avoid full environment-document polling edge-proxy#195 - without shared state, implementing this issue means that each Edge Proxy instance needs a connection to the Core API
• Support persistent identities and traits #14 - whatever persistence layer we arrive at for this issue, will likely be re-usable for the other (or vice versa).