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
Without this, clang complains that we violate argument dependent
lookup rules:
note: 'operator<<' should be declared prior to the call site or in namespace 'sstables'
std::ostream& operator<<(std::ostream&, const sstables::bound_kind_m&);
we can't enforce the #include order, but we can easily move it it
to namespace sstables (where it belongs anyway), so let's do that.
gcc is happy either way.
Deserialisation function returns a structure containing both the value
and its length in the input buffer. In the vast majority of the cases
the caller will already know the length and having this structure will
make it harder for the compiler to emit good code, especially if the
function is not inlined.
In practice I've seen the structure causing register pressure problems
that lead to spilling variables to memory.
