Buf_write should have back-pressure

[talex5]

Buf_write writes allocate extra buffer space as needed. Currently there is no limit to how much. It would be good if it could suspend the calling fiber when some limit is reached and wait until some existing data has been consumed before continuing.

The easiest way would be to limit the number of internal buffers (which only requires checking on the slow allocation path). However, that does expose a detail of the internal buffering to users. Having an exact byte limit would make it easier to specify the behaviour precisely, but would perhaps be more expensive.