Creative: Make dig times near-identical for nodes of all levels

Dig time is modified according to difference (leveldiff) between tool
'maxlevel' and node 'level'. Digtime is divided by the larger of leveldiff
and 1. In creative mode, where the hand is redefined to have maxlevel 3,
this results in higher level nodes taking significantly longer to break.

Now, to speed up digging, hand 'maxlevel' and 'digtime' have been
increased such that nodes of differing levels have an insignificant
effect on digtime. Dig time for all nodes is now identical to that of, for
example, dirt nodes.