Move api/types/client.go and api/types/backend to a more appropriate package

[dnephin]

The api/types package should only contain the types used to serialize/deserialize requests and responses for the HTTP API. This is the case for most of the types in this package and sub packages.

There are two exceptions:

• api/types/client.go contains types that are used by APIClient interface in the client package
• api/types/backend contains types that are used by the backend interfaces between the API routers and the server implementation

These two categories of types are never used for serialization/deserialization.

I propose we move these types to a more appropriate place:

• api/types/client.go should move to client/types
• api/types/backend could be split up and moved to api/server/router/<component> or moved to api/server/backend/types

cc @tiborvass @vdemeester @cpuguy83

[tiborvass]

@dnephin I'm fine with that. Thanks!

[cpuguy83]

api/server/backend/types seems better than api/server/router/ since these are typically shared between the router and the backend.

client/types sounds great.

[vdemeester]

Preparing a few PRs on that. Type aliases (go 1.9) is gonna help tremendously for not breaking people 👼