cxx: simplify basepool locking

Attempt at a further improvement towards fixing #128 (closed):

Instead of using an atomic flag to notify when sockets_ is changed, let's simply store a sockets_empty_ atomic bool that is set every time sockets_ is changed. Also instead of waiting for a signal, let's simply wait 50ms and then check again. This is easier than adding a condition variable.

Worked for me, but needs more testing.