* ec: read a needle's intervals in parallel
A needle spanning more than one EC block gets one interval per block, and
consecutive blocks live on different shards. We read those intervals in
sequence, so a 4MB chunk landing in a volume's 1MB small-block region cost
five round trips to five different servers.
Read them concurrently into disjoint slices of a single buffer, at most 8 in
flight. Same change in the Rust volume server's phase C.
* ec test: seed the random payload instead of the deprecated rand.Read
Chris Lu
51eb5333d3