Make the ItemRepositoryRegistry class a singleton; get rid of locking.

• remove allocateRepository() as we do not have numbered subdirectories

inside the per-session cache location anymore;

• remove locks as the session is already locked very reliably;
• make the ItemRepositoryRegistry a true singleton (private ctor, m_self

field, self() accessor) and make globalItemRepositoryRegistry() a mere
wrapper to ItemRepositoryRegistry::self().

This removed a loop in ItemRepositoryRegistry::open() and thus decreased
nesting level of everything inside the loop, so I took opportunity to
reformat the function body as well.