Unload caches and plugins after dump
Counterintuitively, keeping the cache around not only uses more
memory, but also makes the dump slower. Huh. Who'd have thunk?
Removing the cache altogether gives a noticeable slowdown,
especially when dealing with lots of instances of the same class
like when dumping a container from a busy project.
Making the cache an instance property saves both memory and speed.
Since we used to persist parser plugins in Kint\Kint to keep these
caches alive, removing the plugin pool is the easy way to flush
them (rather than manually flushing the cache on every dump in eg.
setParser) which it turns out _also_ makes things a bit faster.
Strangely enough the same could not be said for Rich\CallablePlugin
which gets slower when cleaning the cache between runs. Perhaps
this is because it's storing whole output strings and saves more
effort? But that doesn't explain why it also saves memory. Oh well.
ParameterHoldingTrait's cache was never even used. Great.