AK: Rename Time to Duration

That's what this class really is; in fact that's what the first line of
the comment says it is.

This commit does not rename the main files, since those will contain
other time-related classes in a little bit.

Kernel/Net/Socket.cpp

@@ -245,7 +245,7 @@ ErrorOr<size_t> Socket::read(OpenFileDescription& description, u64, UserOrKernel
 {
     if (is_shut_down_for_reading())
         return 0;
-    Time t {};
+    Duration t {};
     return recvfrom(description, buffer, size, 0, {}, 0, t, description.is_blocking());
 }