Resolve warnings on NEWCONSENSUS / NEWDESC events

I see hundreds of warnings like this whenever a new consensus arrives in the bandwidth scanner:

WARN[Sat Nov 11 23:03:55 2017]:Need to getinfo ns/id for router desc: 5EED8CE170B8889F6C041C26C934304FCCF02955
WARN[Sat Nov 11 23:03:55 2017]:NS map count of 6618 is below consensus count 6619
WARN[Sat Nov 11 23:03:55 2017]:No router desc for 602239BF846FA9A201E0ADA89FF83DC3E1F9766E after NEWDESC

We should either:

fix the underlying issues that are causing these warnings, or
downgrade them to NOTICE or INFO