Use separator between key fields in cache keys

[BoD]

Currently, when using multiple key fields, they are concatenated without a separator in the cache key, which can lead to collisions, e.g. with extend type User ＠typePolicy(keyFields: "firstName lastName") both users ann acorn and anna corn will have the same key User:annacorn.

Let's use + as a separator, and escape it in the values (see discussion).

Note: this is a breaking change - meaning any existing cache keys with more than 1 key field will become invalid after this change.