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
Instructions retired per op is a much more stable than time per op
(inverse throughput) since it isn't much affected by changes in
CPU frequencey or other load on the test system (it's still somewhat
affected since a slower system will run more reactor polls per op).
It's also less indicative of real performance, since it's possible for
fewer inststructions to execute in more time than more instructions,
but that isn't an issue for comparative tests).
This allows incremental changes to the code base to be compared with
more confidence.
I'd like to add an instructions counter which isn't accessible via
a global, so make the snapshot function a member. Out of respect to #1,
define functions for getting the number of allocations and tasks processed,
as they need heavy header files.
I used {:.0} to truncate to integer, but apparently that resulted
in only one significant digit in the report, so 93.1 was reported as
90. Use the {:5.1f} to avoid truncation, and even get an extra
digit (we can have fractional tasks/op due to batching).
Current result is 93.1 allocs/op, 20.1 tasks/op (which suggests
batch size of around 10).
Closes#8550
Those functions were defined in a header, but not marked inline.
This made including the header from two source files impossible,
as the linker would complain about duplicate symbols.
Rather than making them inline, put them in a new source file
perf.cc as they don't need to be inline.
