Fix event-loop keep-alive behavior with multiple concurrent transfers.
Fix event-loop keep-alive behavior with multiple concurrent transfers.

uv_ref and uv_unref only set and clear a boolean flag, so we have to
implement the counter ourselves.

tessel/node-usb,marcopiraccini/electron-usb,rhuehn/node-usb,heavyk/node-usb,SimplyComplexCo/node-usb,rhuehn/node-usb,torworx/node-usb,nonolith/node-usb,LinusU/node-usb,tessel/node-usb,heavyk/node-usb,nonolith/node-usb,joachimneu/node-usb,marcopiraccini/electron-usb,tessel/node-usb,daiguangbao/node-usb,SimplyComplexCo/node-usb,LinusU/node-usb,CargoSense/node-usb,heavyk/node-usb,CargoSense/node-usb,nonolith/node-usb,torworx/node-usb,torworx/node-usb,daiguangbao/node-usb,nonolith/node-usb,marcopiraccini/electron-usb,stberger3040/node-usb,joachimneu/node-usb,stberger3040/node-usb,LinusU/node-usb,daiguangbao/node-usb,joachimneu/node-usb,stberger3040/node-usb,LinusU/node-usb,tessel/node-usb,CargoSense/node-usb,SimplyComplexCo/node-usb,marcopiraccini/electron-usb,torworx/node-usb,stberger3040/node-usb,daiguangbao/node-usb,stberger3040/node-usb,CargoSense/node-usb,heavyk/node-usb,joachimneu/node-usb,rhuehn/node-usb,torworx/node-usb,SimplyComplexCo/node-usb,rhuehn/node-usb