Fix class path scaning on each deserialization

[kostya05983]

Hello, there is a problem with Json.createPatch(), it calls JsonProvider.provider() on each call.

Accordingly to java doc of provider() method: "Creates a JSON provider object. The provider is loaded using the ServiceLoader.load(Class) method. If there are no available service providers, this method returns the default service provider. Users are recommended to cache the result of this method.".

Let's cache it as static variable.

We faced with a lot of CPU issues because of this method.

What do you think about it?

[cowtowncoder]

Thank you @kostya05983 -- this makes sense.

One quick clerical thing: unless I have asked for & received CLA before, we'd need it from here:

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

and the usual way is to print, fill & sign, scan/photo, email to info at fasterxml dot com.
Once we received it, I can proceed merging the fix.
Note that CLA only needs to be sent once before the first commit; if you have done it before just let me know name under which it was sent.
Either way, going forward sent CLA works so need not be sent again.

Thank you for providing this PR -- looking forward to merging it, for inclusion in 2.16.0!

[kostya05983]

Thank you for your instructions @cowtowncoder, I've signed CLA and sent it to fasterxml dot com. I hope that everything is correct.

[cowtowncoder]

Thank you @kostya05983 CLA was fine! Merging.