Throw away any existing uv_default_loop, recreate after fork

[Spudz76]

(also) Fixes #3455

This is an alternate approach to the problem, it does not function any differently however might be cleaner. Simply destroy any existing default loop to cause a new one to be created on the next uv_default_loop() call (in Signals constructor). This completely avoids using the experimental/non-guaranteed uv_loop_fork() function.

[xmrig]

Is uv_loop_close really needed? It does not look right to close the freshly created default loop and create it again after. Also, the addition of #include <uv.h> is not required in App.h.
Thank you.

[SChernykh]

uv_loop_close will be a no-op at best. If any libuv handle was created at this point, uv_loop_close will fail and return UV_EBUSY error. If no handles were created, there is no point in closing the loop.

[Spudz76]

I agree but it works. So it must be doing something.