Increase backdrop_read workers from 4 to 8
Increase backdrop_read workers from 4 to 8

We're seeing lots of connections occasionally to backdrop_read, the
user agent is node-XMLHttpRequest, which I'm guessing means the
Performance Platform.

The api machines look like they have enough RAM to allow for more
workers, so double the number. This'll help to avoid requests queuing
up during busy periods.

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