StreamMetadata object should also contain the mapping of blockId and its MD5 checksum as a dynamic property

[skmonga]

Currently the StreamMetadata object doesn't contain the mapping of the blockId with its MD5 checksum. This mapping is only maintained at the fog where the stream was registered. As a result, when a client or a fog uses getStreamMetadata(), the object returned doesn't contain this mapping. However this mapping should not be returned to the client directly while it is useful for the fog devices (recovery is one of the possible scenarios where this is useful when a fog device dies). So this mapping should be added to the StreamMetadata object as a dynamic property.