Since commit cc64c715, UpdateService::Enqueue() throws
ProtocolError(ACK_ERROR_UPDATE_ALREADY) instead of returning 0.

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.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

No caller needs this.  Fixes use-after-free after returning from
Client::OnTimeout().

Random access is not necessary here.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

This patch implements a light-weight inotify library, and watches all
directories below the music directory.  It updates all directories
where files changed after a delay of 5 seconds.