Do not try to compile threadpool_common.cc, if threadpool is not available
on current OS.
Also introduced undocumented/uncached CMake variable DISABLE_THREADPOOL,
to emulate builds on exotic OSes.
Remove threads that are doing nothing but wait
- main thread now handles the connections
(if threadpool is used, also threadpool threads would wait for connections)
- thread for socket and pipe connections are removed
- shutdown thread is now removed, we wait for shutdown
notification in main thread as well
- kill_server() is also called inside the main thread, after connection
loop finished.
