Previously, an authority with a clock more than 60 seconds ahead could
cause a client with a correct clock to warn that the client's clock
was behind. Now the clocks of a majority of directory authorities
have to be ahead of the client before this warning will occur.
Relax the early-consensus check so that a client's clock must be 60
seconds behind the earliest time that a given sufficiently-signed
consensus could possibly be available.
Add a new unit test that calls warn_early_consensus() directly.
Fixes bug 25756; bugfix on 0.2.2.25-alpha.
