jsroot: introduce _make_request and _after_request properties

This makes possible to provide special handling for remote objects,
when not the whole object should be fetched to the GUI.
Also in _after_request function one could manipulate object
or just cache results for the next request

Use _after_request in the sniffer for StreamerInfo object to
avoid special handling in JSROOT

Signed-off-by: Bertrand Bellenot <bertrand.bellenot@cern.ch>