THistSvc: cache TObject types

The `TObject::InheritsFrom` call can be quite slow. Since we already
deduce the object type (`TH1`, `TTree`, etc.) during registration, save
this information and use it instead of re-deriving it every time.
Also avoid calling `updateFiles` if no TTrees are registered.

Both these changes significantly speed up the `EndEvent` incident
handling for jobs with large (>10k) number of histograms.

Also re-organize `struct THistID` to avoid padding and delete unused
`reset` method.