Rename one-way CellStuf#send to notify

This both better describes what is happening (no reply expected),
and it avoids naming conflicts with a future patch I am working on.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6727/