[cache] Reimplementation to use the new core caches

[florian-h05]

Fixes #184.

Reference https://github.com/openhab/openhab-core/pull/2887. Reference https://github.com/openhab/openhab-core/pull/3204.

Description

This implements:

• for the current methods of cache:
  • check if the new caches from core are available
  • if they are available, log a deprecation warning and use the shared cache from core instead of the addon‘s shared cache implementation so that we can remove SharedCache from the addon
  • if they are not available (means the library runs on an „old“ openHAB version), do nothing and keep the current behaviour
• add a private property to cache to allow access to the private cache
• add a shared property to cache to allow access to the shared cache

Unit test is updated to test the JSCache class instead of the exported methods of the cache namespace, because testing the exported methods for each case is quite complex and given the fact that the compatibility layer will be removed at some time, I decided to not take that effort.

Testing

The functionality of JSCache is tested by the Jest unit test, the exports are tested by me.

[florian-h05]

@digitaldan @jpg0 @rkoshak Can you please review? This is urgent, because we should get this merged before the sixth milestome and the feature freeze comes on Sunday evening.

You only need to review cache.js and the README changes, the rest is not so important.

[florian-h05]

@digitaldan @jpg0 If you want to give your review, please do it until this evening (+11 hours from now). After this, I'll merge to get this in before the feature freeze; I've already got two reviews so it is not unreviewed.

[jpg0]

LGTM!