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
A set of futures that can be polled.
Polling the set (`poll` function) returns the value of one of
the futures which became available or `std::nullopt` if the given
logical durationd passes (according to the given timer), whichever
event happens first. The current implementation assumes sequential
polling.
New futures can be added to the set with `add`.
All futures can be removed from the set with `release`.
