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
Wokring with collections can be done via const- and non-const
references. In the former case the collection can only be read
from (find, iterate, etc) in the latter it's possible to alter
the collection (erase elements from or insert them into). Also
the const-ness of the collection refernece is transparently
inherited by the returned _elements_ of the collection, so when
having a const reference on a collection it's impossible to
modify the found element.
This patch introduces a immutable_collection -- a wrapper over
a random collection that makes sure the collection itself is not
modified, but the obtained from it elements can be non-const.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
