luci-base: xhr: increase poll request timeout

Some status requests can take quite some time to finish, the LuCI DSL
status information in particular.

Since the polling loop code already takes care of not relaunching
requests which are already running, increase the per iteration timeout
to up to five times the poll interval.

This should be sufficient to let most operations complete.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>