hard limit on max size of any request sent by producer

See discussion on:
https://github.com/tylertreat/mq-benchmarking/pull/2

This probably needs to be configurable and possibly thought through (does it belong somewhere else in the flush path instead/as well?), but it seems to work for now to make the benchmark run.

@wvanbergen @Sirupsen @graemej @burke 
Maybe `BackPressureThresholdBytes`, to more clearly show what the value means? Feel free to disagree.