Turn on many more Eclipse warnings, and fix them

- Warn on empty statements, e.g. "for (;;);". These may be
   typos and are easily replaced by "for (;;) {}" which is more
   explicit.
 - Warn on field hiding. This allows cleanup of many acceptance test
   members, at the cost of a couple of renames and the occasional
   suppression (when the field is in a public nested enum that shadows
   a public constant).
 - Warn on unnecessary casts.
 - Warn on unused declared thrown exceptions. In addition to reducing
   method signature length and number of imports, this also eliminated
   some impossible catch blocks.
 - Warn on missing @Override annotations.
 - Warn on unused parameters. This is likely the most controversial,
   as a few relatively common patterns require unused parameters in a
   way that Eclipse can't ignore. However, it also resulted in cleanup
   of a lot of unnecessary injections and method parameters, so I
   think the cost was worth it.

Change-Id: I7224be8b1c798613a127c88507e8cce400679e5d

gerrit-sshd/src/main/java/com/google/gerrit/sshd/NoShell.java

@@ -54,6 +54,7 @@ class NoShell implements Factory<Command> {
     this.shell = shell;
   }
 
+  @Override
   public Command create() {
     return shell.get();
   }
@@ -77,27 +78,33 @@ class NoShell implements Factory<Command> {
       this.sshScope = sshScope;
     }
 
+    @Override
     public void setInputStream(final InputStream in) {
       this.in = in;
     }
 
+    @Override
     public void setOutputStream(final OutputStream out) {
       this.out = out;
     }
 
+    @Override
     public void setErrorStream(final OutputStream err) {
       this.err = err;
     }
 
+    @Override
     public void setExitCallback(final ExitCallback callback) {
       this.exit = callback;
     }
 
+    @Override
     public void setSession(final ServerSession session) {
       SshSession s = session.getAttribute(SshSession.KEY);
       this.context = sshScope.newContext(schemaFactory, s, "");
     }
 
+    @Override
     public void start(final Environment env) throws IOException {
       Context old = sshScope.set(context);
       String message;
@@ -115,6 +122,7 @@ class NoShell implements Factory<Command> {
       exit.onExit(127);
     }
 
+    @Override
     public void destroy() {
     }
   }