Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
Instead of playing the game of casting between stream_event and derived
class. We overload handle_stream_event with derived stream_event class.
virtual void handle_stream_event(session_complete_event event) {}
virtual void handle_stream_event(progress_event event) {}
virtual void handle_stream_event(session_prepared_event event) {}
Also, make the virtual function non pure virtual, so user can override
the interested event only without defining all of the three.
