Optimize peer I/O:

- Limit the lifetime of a buffer that was only used in the early
  phases of peer connection establishment but which lived on as
  long as the peer was active.
- Cache the message used to transfer manifests, so it can be reused
  instead of recreated for every peer connection.
- Improve the reading of partial messages by passing a hint to the
  I/O layer if the number of bytes needed to complete the message
  is known.

src/ripple/app/misc/impl/Manifest.cpp

@@ -431,6 +431,9 @@ ManifestCache::applyManifest(Manifest m)
         iter->second = std::move(m);
     }
 
+    // Something has changed. Keep track of it.
+    seq_++;
+
     return ManifestDisposition::accepted;
 }