Change LogReader API to be able to replace messages

The mutation API in LogReader was not able to express dropping messages,
or growing messages.  This enables more aggressive mutation.

Change-Id: I477482da4262483a780d15ebf8c98a51e37099f6
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/containers/resizeable_buffer.h b/aos/containers/resizeable_buffer.h
index 484d93e..664fa2f 100644
--- a/aos/containers/resizeable_buffer.h
+++ b/aos/containers/resizeable_buffer.h
@@ -55,6 +55,8 @@
   size_t size() const { return size_; }
   size_t capacity() const { return capacity_; }
 
+  bool empty() const { return size_ == 0; }
+
   void reserve(size_t new_size) {
     if (new_size > capacity_) {
       Allocate(new_size);