Since http2 spec defines the receive windows's size and max size of
frame in the stream, the underlayer - gRPC client can pre-read data
from server even if the application layer hasn't read it yet.
And the initialized cluster has 20KiB snapshot, which can be pre-read
by underlayer. We should increase the snapshot's size, just in case
that the io.Copy won't return the canceled or timeout error.
Fixes: #14477
Signed-off-by: Wei Fu <fuweid89@gmail.com>
They used to take >10min with coverage, so were causing interrupted
Travis runs.
Know thay fit in 100-150s (together), thanks also to parallel
execution.
