Make http_proxy generic based on StoreType and make backing_store_ a member
Need virtual dtor to compile under g++-4.9
Throw in dummy implementations for local proxy so that it compiles
generic record_process(): takes a TargetType & a parameter pack for TargetType
Modify driver programs based on new process_recorder.{hh,cc}
Remove BackingStore base class; we now use templates instead of inheritance
Rename backing_store.{cc,hh} to http_disk_store.{cc,hh}