R1:bf3879b1c72f

Fully fix a bad rule object aliasing issue custom remarkup rules

Summary:
Fixes T10234. This is a more thorough fix.

Root issue is that some time around D13589, we started hitting an object cache for `loadCustomInlineRules()`, but didn't adjust the code to account for that.

So if a page created multiple similar engines, we'd return the same `$rule` object for multiple engines, call `setEngine()` on it with different engines, and then possibly try to render using an already-expired engine the second time…