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 updateable_value and updateable_value_source classes allow broadcasting
configuration changes across the application. The updateable_value_source class
represents a value that can be updated, and updateable_value tracks its source
and reflects changes. A typical use replaces "uint64_t config_item" with
"updateable_value<uint64_t> config_item", and from now on changes to the source
will be reflected in config_item. For more complicated uses, which must run some
callback when configuration changes, you can also call
config_item.observe(callback) to be actively notified of changes.
