Big documents are slow

[hamiltop]

@samginn I'm opening this issue to explore the big document perf in https://github.com/devinus/poison/issues/90

[hamiltop]

Looks like this is just some network tuning. If I set the buffer on the socket to 1M, it goes plenty quick.

I'm not sure about the tradeoffs involved with a large buffer. The easiest solution would be to expose the socket opts and allow you to pass them in on start_link.