Remove our dependency on async_extra

[brendanlong]

Right now we use Async.Socket from async_extra, which means we have to build all of core. If we could replace that one module with something else, pgx_async would only need core_kernel, core_unix, async_kernel and async_unix.

[brendanlong]

Actually maybe it's not Socket. I'll investigate more. I seem to remember there being one thing async_extra or async_extended that we use.

[brendanlong]

Oh, it's Tcp.

But looking at it more, it looks like async_extra isn't that bad. async_unix depends on core, so there's not much we can do to remove this dependency.