Limit number of concurrent writes while syncing #6541
Description
Currently we spawn a goroutine to pull a block, and that goroutine does the file write when it's done. The number of goroutines is limited by the puller max pending kib setting, but this can still be hundreds of goroutines if the requested blocks are small. This can result in hundreds of concurrent disk writes, in turn spawning hundreds of I/O bound threads.
We should have a couple of goroutines responsible for the writes to handle this separately from the pullers.