G_IO_PRI is not supported

[mzabaluev]

libuv does not support polling for "urgent data". The need for this is marginal, but in case there is any GLib code that relies on G_IO_PRI events, the uv backend has to support it.

One solution is to have a helper thread per each context that's got a G_IO_PRI descriptor added. The thread would poll for events not supported by libuv, doing it concurrently with the polling in libuv, unblocking immediately (e.g. via eventfd) when the libuv poll has unblocked and vice versa.