feat(Sockets.jl): accept as many connections as possible in one loop

[pankgeorg]

This PR introduces (another) buffer, on top of LibUv's 511 connection limit. This buffer aggressively accepts new connections if they are there (with accept_nonblock) and even rejects some of them (this shouldn't exactly happen like that, as TCP doesn't mean HTTP, but this is a demo). The performance difference is negligible (I measure +1% for this PR with a yielding handler, but I expect this to be much more in computation intensive code, and, more importantly, I expect this to handle spikes of traffic better)

WARNING: NOT TESTED, NOT THREADSAFE (...insert any disclaimer here...)

Inspired by libuv/libuv#3071 where it says

since uv_accept doesn't actually accept anything, but merely moves the already accepted connection to a new handle. Perhaps both the silent acceptance of connections and the automatic disabling of listening should be mentioned in the uv_listen, uv_accept and uv_connection_cb documentation entries.