Tweaked multi-break parameters a bit (to make building less painful)

After building for a while, people are reporting that misplacing blocks is a bit painful.  So for now, I've halved the following parameters:

1. # of breaks required for lockables, such as chests
2. the construction-grade modifier; now stone bricks, for instance, will be easier to break

This entire process is going to require a lot of back-and-forth, as we wish to strike a balance between creation and destruction.  But for now it seems that this change will make play more enjoyable without impacting how challenging it is too much.