Improve tolerance for dirauths with skewed clocks

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.

changes/bug25756

@@ -0,0 +1,7 @@
+  o Minor bugfixes (error reporting):
+    - Improve tolerance for directory authorities with skewed clocks.
+      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.  Fixes bug 25756; bugfix on 0.2.2.25-alpha.