Whether a server can vote in a Raft configuration is not part of the
address. `server_address` was used in many context where `can_vote` is
irrelevant.
Split the struct: `server_address` now contains only `id` and
`server_info` as it did before `can_vote` was introduced. Instead we
have a `config_member` struct that contains a `server_address` and the
`can_vote` field.
Also remove an "unsafe" constructor from `server_address` where `id` was
provided but `server_info` was not. The constructor was used for tests
where `server_info` is irrelevant, but it's important not to forget
about the info in production code. The constructor was used for two
purposes:
- Invoking set operations such as `contains`. To solve this we use C++20
transparent hash and comparator functions, which allow invoking
`contains` and similar functions by providing a different key type (in
this case `raft::server_id` in set of addresses, for example).
- constructing addresses without `info`s in tests. For this we provide
helper functions in the test helpers module and use them.
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
