A worker pool is only needed for rpc instances that serve call backs
Since the glib-2.0 patch
4d2e77a55 GThreadPool: Always use the thread-spawning thread for the global shared thread pool
https://github.com/GNOME/glib/commit/4d2e77a5546cccc10f83d103d6f1527a5c9bcf13
creating a threadpool spawns a new thread. Previously, this was only the
case for non-Linux platforms.
This can cause issues for daemons that fork and depend on libraries that
load apteryx on LT_INIT, as the glib threadpool spawned thread will not
be copied on the fork.
Additionally, now libraries that only use get calls will be running an
extra thread from the threadpool.
Move the threadpool creation into request_cb() to only spawn the pool
managing thread for applications that require callbacks using workers.
