TCP connections should be persistent (not closed after each batch)

[parasj]

Based on my reading of the code, it looks like the connection isn't persistent --- the destination port is left listening, but a separate connection is still used for each batch of chunks.

I'm approving since it improves bandwidth, but I still think that keeping the TCP connection itself alive for a long time is good, to avoid ramping up congestion control each time.

Originally posted by @samkumar in #38 (review)