Add panes and data attached to them to a new mempool

Add panes and data attached to them to a new mempool

The 'pane' mempool is for allocations that scale roughly
with the number of panes.
The manage the ->data nicely, 'pane_register()' is now a varadic
function where the 'data' should be left off rather than having
'NULL' be explicitly passed.

Signed-off-by: NeilBrown <neil@brown.name>