eliminated unused ServiceRegistryImpl.install() method
micro optimization - eliminating ServiceRegistryImpl.isRemoved() method to prevent reetrant locking
take ServiceRegistryImpl internal state into account when creating/reverting snapshots
Revisited takeSnapshot() & revert() method modifiers
 * eliminated protected and public modifiers - both methods are package private
 * eliminated synchronized keyword on derived classes - it's not needed because TransactionalObject base class takes care of proper locking