Refactor the reader/dispatcher

The previous design did not handled errors correctly and was kind of
spaghetti code. With the new one, the reader and the dispatcher are
clearly separated. The filter will only exit on an error from the reader
or if EOF has been reached, any other error is displayed but does not
exit the filter, which is required by the API. If the filter must exit,
all threads are gracefully stopped.

examples/dummy.rs

@@ -1,6 +1,3 @@
 fn main() {
-    match opensmtpd::dispatch() {
-        Ok(_) => {}
-        Err(e) => eprintln!("Error: {}", e.as_str()),
-    }
+    opensmtpd::run();
 }