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().

BufferedSocket has just an input buffer, and FullyBufferedSocket adds
the output buffer.

Don't use a global loop over the whole client list.

Use std::list instead of GList.

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.

The soure file client.c has nearly 1000 lines, time for splitting it
into smaller pieces to improve readability.