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
Using these instead of using the absl headers directly adds support
for heterogeneous lookup with sstring as key.
The is no gain from having the hash function inline, so this
implements it in a .cc file.
Signed-off-by: Rafael Ávila de Espíndola <espindola@scylladb.com>
