Alternative entity identifier transport for GET evaluation request

[iamafanasyev]

In addition to preloading idea (of GET-eval-request) from #613 we could consider an exclusion entity identifier from URL, so it could be safely cached in the server response (e.g. link tag in HTML or link response header). To do so we could parametrise Flagr instance with cookie name to consider as an alternative (implicit) entity identifier provider to the server. Good questions are:

• should we mirror the missing-entity-id-in-POST-request semantics for no-cookie-and-no-query-parameter-situation? (probably yes)
• should we "generate"/"issue" (i.e. respond with Set-Cookie) a new entity identifier for the request? (probably configurable via Flagr instance parametrisation)
• should we consider any kind of explicit cache control configuration for GET-eval-requests?

What are your thoughts?

[iamafanasyev]

Well effectively it makes possible to omit entityID (id in case of the current GET-proposal) in any evaluation request, however:

• it makes sense almost exclusively for GET-requests (as it mostly addresses caching conserns);
• it doesn't fit batch evaluation perfectly, as (i guess) initially batch evaluation assumes flag evaluation using independent entity contexts (including different entity identifiers in general), so implicit universal entity identifier (from cookie) for several requested entity contexts doesn't make much sense