store: Exported the store base object type as opaque type.

Renamed from store_obj_t to sdb_store_base_t to avoid a name-clash with
sdb_store_obj_t.
utils llist: Added TODO note about iterator improvements.
store: Moved object type attribute to the base object.
store: Introduced sdb_store_host_tojson() to serialize a single host.
store: Added flags to JSON functions indicating information to leave out.

This may be used to leave out services or attributes from the serialized
object. For this purpose, the serialization function has been simplified to
handle attributes and services mostly alike.
store_test: Test various serialization flags.