R1:85ff1d5c2df0

Reduce the impact of `bin/storage dump`

Summary:
Ref T12646.

- Use "wb1" instead of "wb" to use level 1 gzip compression (faster, less compressy). Locally, this went about 2x faster and the output only grew 4% larger.
- LinesOfALargeExecFuture does a lot of unnecessary string operations, and can boil down to a busy wait. The process is pretty saturated by I/O so this isn't the end of the world, but just use raw ExecFuture with FutureIterator so that we wait in `select()`.
- Also, nice the process…