PREPARE": rename members, methods, classes to follow the spec 
(a code review request)


sql/mysql_priv.h:
  enum_metadata_type -> enum_table_ref_type
sql/sp_head.cc:
  Metadata_version_observer -> Reprepare_observer
sql/sql_base.cc:
  metadata -> table_ref
sql/sql_class.cc:
  Replace an abstract interface with a concrete implementation.
sql/sql_class.h:
  enum_metadata_type -> enum_table_ref_type
sql/sql_prepare.cc:
  Move implementation of Execute_observer to sql_class.cc and
  rename the class to Reprepare_observer.
  Use getters instead of direct access to the members.
sql/table.h:
  metadata -> table_ref