Without IdleTimeout, clients holding keep-alive connections open indefinitely
caused server-side goroutines (and their ~16KB of buffers) to accumulate
linearly until OOM.
Reduces IdleConnTimeout to 5 seconds. Reverse proxying to a server that
has a shorter keep-alive may cause "EOF" and "connection reset by peer"
issues as the connections may be closed by the upstream before our
client notices.