Avoid deadlocking on log scanning with lots of output on stderr #5738
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When we're running our log scanning code, we can end up with a deadlock if there is lots of output on standard error because we either don't read standard error at all or we only read it when we're done reading standard output. To fix this, let's add and use some functions that either discard standard error if we're not using it, as well as making sure we read standard error in a separate goroutine so the child process doesn't block.
Fixes #5720