NO-JIRA: Minor refactor to improve code safety: calling shared_from_this on creation.

Previous anti-pattern: Classes need to call shared_from_this during creation,
but can't call it in the ctor so had a separate initiailize function that the
user was required to call immediately after the constructor. Possible for user
to forget.

Improved pattern: Introduce public static create() functions to call constructor
and initialize, make constructor and initialize private.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1561828 13f79535-47bb-0310-9956-ffa450edef68