exec: interleave stdout/stderr logging
exec: interleave stdout/stderr logging

The io.MultiReader(r1, r2) reads r1 until EOF before moving on to r2.
When used for stdout/stderr stderr will not be written to the log until
stdout reaches EOF. This patch reads stderr in a go routine so that the
two streams can be interleaved properly.

Fixes: #2479

Signed-off-by: Noah Watkins <86ecd7d1c53cf33a85014f7cab78876cb4491020@gmail.com>

rook/nfs,rook/rook,rook/nfs,rook/rook,rook/rook,rook/rook,rook/rook,rook/nfs