Windows: Larger number of concurrent receives causes intermittent blocking

[eile]

Originally reported by @delyas as Eyescale/Equalizer#265:

when having multiple concurrent input streams, the recv thread blocks 200-2000ms on a single receive. The current working theory is that a partial package is received, and the kernel does not receive more data on this socket since others go unprocessed, until some internal timeout is reached.

Implement threadpool-based receiving to see if that fixes the issue.