Cache when there is no metadata for a class

[adrienbrault]

What this changes:

• In debug, the factory will try to load the metadata of a class only once (per request) if there is none, instead of multiple times
• In debug=false, the factory will try to load the metadata of a class only once until the cache is cleared (among multiple requests)

There should be way less file_exists going on :)

[adrienbrault]

FYI benchmarking with a vagrant box an api endpoint serializing thousands of objects:

	enabled	disabled
dev	2.8s	5.7s
prod	2.4s	5.6s

@schmittjoh I know that the patch isnt sexy but WDYT ?

[schmittjoh]

I wasn't really sure whether it was worth it when I saw it, and then I more or less forgot this PR.

We will probably need to refactor the factory soon, but I'll merge this now as the improvement seems to be quite significant for some systems as you pointed out.