Skip performance tests when in regular mode
Common caching support in the Nitrate class

Caching now handled in Nitrate instead of separate implementations
New caching class methods: _cache_lookup() and _is_cached()
Attribute initialization moved to a separate _init() method
Many adjustments in handling object fetching and initialization
A bunch of new test cases covering the caching functionality

Signed-off-by: Petr Šplíchal <psplicha@redhat.com>