NEXUS-6851: Change type of field "raw" to binary

As yourkit showed a lot of string manipulation (and hence
creation of intermediate char and string objects) during
serve of package metadata.

The "raw" field contains complete metadata JSON as string, and
hence can be bigger than few bytes.

This change _removes_ the offending classes from stack trace (calls)
OStringSerializerHelper and OBinaryProtocol.