cells: Drop support for auto starting CellAdapter

Starting CellAdapter from its constructor suffers from the mistake of leaking a
reference to CellAdapter (through CellNucleus) before it is constructed.  This
patch removes support for starting the CellAdapter in its constructor - most
subclasses didn't make use of this feature anyway.

In the current patch subclasses call start instead, which obviously suffers from
the same problems as if CellAdapter had called it. Subsequent patches will
address that problem.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7514/