Cothread: Add mechanism for requesting entire waveform

As ca{get,monitor}(pv, ..., count=0) requests data dependent waveform
length we need a simple mechanism for always requesting the entire
waveform.

Also update the documentation for count.

git-svn-id: svn+ssh://serv0002.cs.diamond.ac.uk/home/subversion/repos/controls/diamond/trunk/python/cothread@112382 e099a375-04f9-0310-9d5f-a741eaff62e1