Changing the key for cached entry

Add :memento.core/key-fn to cache or mount config (or use mc/key-fn value) to specify a function with which to manipulate the key cache will use for the entry.

Example building on previous suggested cache/inf cache configuration:

(defn get-person-by-id [db-conn account-id person-id] {})

; when creating the cache key, remove db connection
(m/memo #'get-person-by-id (assoc cache/inf-cache mc/key-fn #(remove db-conn? %)))
; or more explicit
(m/memo #'get-person-by-id {mc/type mc/guava mc/key-fn #(remove db-conn? %)})

When creating the cache key, remove db connection, so the cache uses [account-id person-id] as key. Thus calling the function with different db connection but same ids returns the cached value.

Another example:

(defn return-my-user-info-json [http-request]
  (load-user (-> http-request :session :user-id)))

;; clearly the cache hit is based on a deeply nested property out of a huge request map
;; so we want to use that as basis for caching
(m/memo #'return-my-user-info-json (assoc cache/inf-cache mc/key-fn #(-> % first :session :user-id)))

This is both a mount conf setting, and a cache setting. The obvious difference is that specifying key-fn for the Cache will affect all functions using that cache and in mount conf, only that one function will be affected. If using 2-arg memo, then this setting is applied to mount conf.