Reject directory circuits early if they are too skewed.

If we get a connection to a directory that tells us that it's a month in the future, any consensus that it believes in will almost certainly be too new for us to actually use.

(Similarly, if the directory says it's a month in the past, any consensus it believes in will almost certainly be too old for us to use. But we already avoid getting such consensus documents by setting a reasonable If-Modified-Since time.)

We could actually tolerate even more skew than we do with this branch, but IMO it's not a great idea to do so.

This branch is based on top of !500 (merged); only the final 3 commits are new. It's probably best to review !500 (merged) first.

Closes #466 (closed)

crates/tor-dirclient/src/err.rs

@@ -72,6 +72,13 @@ pub enum RequestError {
     /// Unrecognized content-encoding
     #[error("Unrecognized content encoding: {0:?}")]
     ContentEncoding(String),
+
+    /// Too much clock skew between us and the directory.
+    ///
+    /// (We've givin up on this request early, since any directory that it
+    /// believes in, we would reject as untimely.)
+    #[error("Too much clock skew with directory cache")]
+    TooMuchClockSkew,
 }
 
 impl From<TimeoutError> for RequestError {
@@ -147,6 +154,7 @@ impl HasKind for RequestError {
             E::HttparseError(_) => EK::TorProtocolViolation,
             E::HttpError(_) => EK::Internal,
             E::ContentEncoding(_) => EK::TorProtocolViolation,
+            E::TooMuchClockSkew => EK::TorDirectoryError,
         }
     }
 }