For efficient traversal within one partition, e.g. for distributing
partition-local idle events.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Eliminate extra allocations for the std::list node instances.

Publish the Client API, preparing to move more code into the Client
class.

More preparations for multi-player support.

Use std::list instead of GList.