Handle hangup logic in the Stasis message bus and consumers of Stasis messages

This patch does the following:
* It adds a new soft hangup flag AST_SOFTHANGUP_HANGUP_EXEC that is set when a
  channel is executing dialplan hangup logic, i.e., the 'h' extension or a
  hangup handler. Stasis messages now also convey the soft hangup flag so
  consumers of the messages can know when a channel is executing said
  hangup logic.
* It adds a new channel flag, AST_FLAG_DEAD, which is set when a channel is
  well and truly dead. Not just a zombie, but dead, Jim. Manager, CEL, CDRs,
  and other consumers of Stasis have been updated to look for this flag to
  know when the channel should by lying six feet under.
* The CDR engine has been updated to better handle a channel entering and
  leaving a bridge. Previously, a new CDR was automatically created when a
  channel left a bridge and put into the 'Pending' state; however, this
  way of handling CDRs made it difficult for the 'endbeforehexten' logic to
  work correctly - there was always a new CDR waiting in the hangup logic
  and, even if 'ended', wouldn't be the CDR people wanted to inspect in the
  hangup routine. This patch completely removes the Pending state and instead
  defers creation of the new CDR until it gets a new message that requires
  a new CDR.

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@393777 f38db490-d61c-443f-a65b-d21fe96a405b