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
Different request types have different timeouts (for example,
read requests have shorter timeouts than truncate requests), and
also different request sources have different timeouts (for example,
an internal local query wants infinite timeout while a user query
has a user-defined timeout).
To allow for this, define two types: timeout_config represents the
timeout configuration for a source (e.g. user), while
timeout_config_selector represents the request type, and is used
to select a timeout within a timeout configuration. The latter is
implemented as a pointer-to-member.
Also introduce an infinite timeout configuration for internal
queries.
