Improving handling of a trickle of data into `tyger buffer write`

[johnstairs]

By default, tyger buffer write writes 4MB blocks at a time. If its input stream writes, say, 1KB/s, it will take more than an hour to write its first block. On the read end, there will be no data to process until this first block is written, which is unacceptable for any scenario that requires low latency.