Cleaned up ConnStateData's closing and destruction.

Cleaned up ConnStateData's closing and destruction.

1) Despite its name and the "if (open) close" use in ConnStateData destructor,
ConnStateData::close() was not closing anything. It was called from the Comm
close handler and from the destructor and would attempt to immediately delete
the ConnStateData object. Protecting code in deleteThis() may have prevented
the actual [double] delete from happening, but it is difficult to say exactly
what was going on when close() was being called from the destructor.

I converted ConnStateData::close to swanSong, which is the standard AsyncJob
cleanup method. As before, the method does not close anything (which may be
wrong). The swanSong method is never called directly by the user code. It is
called by lower layers just before the job is destroyed.

We may need to add Comm closing code to swanSong. For now, the updated
ConnStateData destructor will warn if ConnStateData forgot to close
the connection. The destructor will also warn if swanSong was not called,
which would mean that the job object is being deleted incorrectly.

2) Polished ClientSocketContext::writeComplete to distinguish
STREAM_UNPLANNED_COMPLETE from STREAM_FAILED closing state. This helps when
looking at stack traces.

3) Added an XXX comment about duplicated code.

4) Documented ClientSocketContext::initiateClose purpose and context.