[entropy.server] do not force disable xcache

This hopefully makes possible to use EntropyCacher in entropy.server
as well.
[entropy.cache] EntropyCacher: promote methods to @classmethod
[entropy.const] const_get_caller(): make it work inside test modules
[entropy.server] add _cache_prefix()

_cache_prefix() will be used for computing a fixed and function
dependent cache key prefix that contains the module name, class name
and method name (retrieved via const_get_caller()).
[entropy.server] injected_library_dependencies_test: add disk caching

The test phase is called frequently in both commit and push phases.
However, between commit and push there are no changes to the repository
database content and caching out this test greatly improves speed.