tdb: use same expansion factor logic when expanding for new recovery area.

tdb: use same expansion factor logic when expanding for new recovery area.

If we're expanding because the current recovery area is too small, we
expand only the amount we need.  This can quickly lead to exponential
growth when we have a slowly-expanding record (hence a
slowly-expanding transaction size).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>