these unused includes are identified by clang-include-cleaner.
after auditing the source files, all of the reports have been
confirmed.
please note, since we have `using seastar::shared_ptr` in
`seastarx.h`, this renders `#include <seastar/core/shared_ptr.hh>`
unnecessary if we don't need the full definition of `seastar::shared_ptr`.
so, in this change, all the unused includes are removed. but there are
some headers which are actually used, while still being identified by
this tool. these includes are marked with "IWYU pragma: keep".
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
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
