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
While a managed_ref emulates a reference more closely than it does
a pointer, it is still nullable, so add a get() (similar to
unique_ptr::get()) that can be nullptr if the reference is null.
The immediate use will be mutation_partition::_static_row, which
is often empty and takes up about 10% of a cache entry.
