Raise guard threshold to top 25% or 2000 kilounits

Authorities now assign the Guard flag to the fastest 25% of the
network (it used to be the fastest 50%). Also raise the consensus
weight that guarantees the Guard flag from 250 to 2000. For the
current network, this results in about 1100 guards, down from 2500.
This step paves the way for moving the number of entry guards
down to 1 (proposal 236) while still providing reasonable expected
performance for most users.

Implements ticket 12690.