Increase priority of Sysctl::Parameters['big rmem']

Sysctl::Parameters['big rmem'] and Sysctl::Parameters['wikimedia base'] both
set net.core.rmem_max, but it's the former that is more specific and that
should therefore win, so increase its priority to 70.

Change-Id: I50d929847dfe0c8caed1818d7ae0e73997e5d9e9