R1:eaa60334ec16

Limit the read buffer size in `bin/storage dump`

Summary:
Ref T13328. Currently, we read from `mysqldump` something like this:

```
until (done) {
for (100 ms) {
mysqldump > in-memory-buffer;
}

in-memory-buffer > disk;
}
```

This general structure isn't great. In this use case, where we're streaming a large amount of data from a source to a sink, we'd prefer to have a "select()"-like way to interact with futures, so our code is called after every read (or maybe once some small buffer fills up…