Properties of specific types of mature RNA products such as microRNAs
are stored in the database as attributes (so that we needn't add
type-specific columns to the rnaproduct table) but have shorthand
accessors such as MicroRNA::arm() backed by cache member variables
in the API (for convenience and speed). In order for this to work
properly we have to push possible changes to the cache variables back
to attributes before they are stored in the database - which is what
this method does.

The synchronisation code itself is generic and has been implemented
in the superclass. Specialisation is handled by class-specific mappings
between member names and respective Attribute codes, provided
by RNAProductTypeMapper.

Limitations:
 - at present there can only be either zero or one target attribute;
   having many attributes with the same code results in an exception
   because we do not know which one(s) to update. In the future we might
   have to extend the mapping syntax to support multiple attributes, for
   now however (i.e. for microRNA arms) we do not need it;
 - chances are things will break badly if we fetch a MicroRNA object
   from the database, change the arm and push it back. Right now this is
   not a problem because RNAProducts do not support database updates at
   all, then again should we want to support them we will have to check
   how AttributeAdaptor handles updates of existing objects (as well as
   the same for DBEntryAdaptor, as a matter of fact).