- hide the details of the Session class using pimpl (mainly so that we can
  easily replace the internal implementation with boost::asio, etc)
- make the Session un-copyable (not absolutely necessary, but possible, and
  it will make other things easier)
- make establish() exception safe
- corrected return value check for socket(2)
- other minor style fixes


git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@1210 e5f2f494-b856-4b98-b285-d166d9295462