Poll usage: Hangup on pending write and Buffer Limits

[mitsuhiko]

Currently our way to invoke poll is pretty hacky and it might cause us problems in two situations:

• If we write to the buffer limit we might have to read first to get more. I don't see how that is in general possible with redis as redis cannot start processing unless it got the entire pipeline, but i need to verify that this is not an issue with writing to different redises
• I think we might not support a hangup while writing. Need to verify this.

[mitsuhiko]

I fixed the second part of it. Need to see if that helps some.