Improve bandwidth usage during gcs uploads
Improve bandwidth usage during gcs uploads

Setting the writer’s ChunkSize to a fairly small value makes things
work better in the presence of limiting upload bandwidth. With it
unset, the writer would first request the whole file contents (which
the rate limiting code only doled out slowly) before uploading, at
which point it would upload everything, thus switching between periods
of using no upload bandwidth and periods of potentially using more than
the rate-limiting code expected.

mmp/bk,mmp/bk