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
The new collector parameter is a pointer to a
`compaction_garbage_collector` implementation. This collector is passed
all atoms that are expired and would be discarded. The body of
`compact_and_expire()` was changed so that it checks cells' tombstone
coverage before it checks their expiry, so that cells that are both
covered by a tombstone and also expired are not passed to the collector.
The collector param is optional and defaults to nullptr. To accommodate
the collector, which needs to know the column id, a new `column_id`
parameter was added as well.
