vote AuthDirMaxServersPerAddr in consensus params

Directory authorities now include their AuthDirMaxServersPerAddr
config option in the consensus parameter section of their vote. Now
external tools can better predict how they will behave.

In particular, the value should make its way to the
https://consensus-health.torproject.org/#consensusparams page.

Once enough dir auths vote this param, they should also compute a
consensus value for it in the consensus document. Nothing uses this
consensus value yet, but we could imagine having dir auths consult it
in the future.

Implements ticket 40753.