tahoe-chk

Recall that "share hashes" are actually poorly named "share tree hashes".
Each is the root hash of a "share hash tree".

Previously we used the number of segments in a share to compute this.
However, there is one "share [tree] hash" *per share* not *per segment*.
After this change, we use the total number of shares instead and this lets us
calculate the correct number of needed hashes of this type.