libuv based event loop / server

[DerThorsten]

based on #620, but #620 was in a strange "broken" state:

• it used xeus::make_xserver_uv_shell_main but in xeus-uv only xeus::make_xserver_uv exists.
• it includes non-existing headers from the wrong repo: #include "xeus-zmq/xhook_base.hpp" is included, but this header is in xeus-uv not xeus-zmq