common: Add SerializableRange class

This class in an extension to the serialization framework. It is designed
to provide an efficient way to serialize a range of elements from some
container, eg. a vector.
mount: Refactor code creating CLTOCS_WRITE message

Refactor code creating CLTOCS_WRITE message using the new serialization
framework. This commit doesn't chagne the behavior of the system, it's
just an example of using the new functions.