dir auths now omit Measured= if rs->is_authority

dir auths now omit Measured= if rs->is_authority

Directory authorities stop voting a consensus "Measured" weight
for relays with the Authority flag. Now these relays will be
considered unmeasured, which should reserve their bandwidth
for their dir auth role and minimize distractions from other roles.

In place of the "Measured" weight, they now include a
"MeasuredButAuthority" weight (not used by anything) so the bandwidth
authority's opinion on this relay can be recorded for posterity.

Resolves ticket 40698.