Use a ring buffer to reduce memory churn

[hadley]

Fixes #704

Initial benchmark indicates that there's more work to be done:

  expression                min   median `itr/sec` mem_alloc
  <bch:expr>           <bch:tm> <bch:tm>     <dbl> <bch:byt>
1 stream_data(req)        6.83s    6.83s     0.147   43.03MB
2 stream_data_raw(req)    5.38s    5.38s     0.186    1.28MB

Also need to figure out why snapshot is broken.