add LRU cache for non-reactive subs

[dvingo]

Spotted the idea here:

day8/re-frame#680 (comment)
can make it configurable by set!ing the cache fn.

subscriptions/src/main/space/matterandvoid/subscriptions/impl/subs.cljc

Line 38 in 1cf6c89

(when (and (ratom/reactive-context?) reaction-or-cursor)