- use forward declarations instead of including headers when possible
- partly as a result of that, moved method definitions from .h to .cc unless
  it does very trivial thing and is deemed to be performance sensitive.
- avoid 'using namespace' in header files
- made data-source related objects non-copyable as much as possible
- fixed a bug of an uninitialized variable
- made coding style more consistent


git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@1110 e5f2f494-b856-4b98-b285-d166d9295462