[os-failedrequests] Implements failedrequests ls --summary

scheduler/OStoreDB/OStoreDB.cpp

scheduler/OStoreDB/OStoreDB.hpp

scheduler/OStoreDB/OStoreDBFactory.hpp

scheduler/Scheduler.cpp

scheduler/Scheduler.hpp

scheduler/SchedulerDatabase.hpp

xroot_plugins/XrdCtaFailedRequestLs.hpp

xroot_plugins/XrdSsiCtaRequestMessage.cpp