Restructure resolver code:
* No weak values to avoid frequently researches of classpath
* restructure resolver of context- and class-classloader
* some minor improvements.
Use SoftReferences instead of WeakReferences to hold the Provider classes. If one provider of the set gets collected by the garbage collector we refetch the provider.
Make build java 6 compatible again
Add test for BackendProviderSet that checks the handling when a SoftReference loose the value due garbage collection.
Merge branches 'ReviewAndReworkBackendProviderResolver' and 'master' of github.com:holisticon/tracee